This will get long again, my apologies. I'll start with the bottom line... it's not too hard to make at least
some changes if you're willing spend a little time learning how. It's not as complicated as I'm sure I'll make it sound.
It can range from very easy to frustratingly difficult to fix issues with the AI's line, depending on what causes the problems. From the looks of that video it might not be too difficult but you never know until you get into it. Very often the AI will just have a really poor line or no line at all on an otherwise perfectly well made track. Some people may get frustrated or bored trying to make it work right, others may just not care since they figure no one would ever bother playing rFactor with the AI. Whatever the reason, it's good to know how to fix them since sometimes it's really simple stuff that isn't done like not even giving them a basic racing line, or not giving them an estimate of fuel usage or not telling them to slow down before the pit lane speed limit line.
Anyway, on to how to do it. DISCLAIMER: I'm obviously no expert, but I've managed to learn how to make a lot of corrections and make workable AIW files from scratch. The first thing is that you need to download a few files. You need the AIW editor itself as well as a developer toolkit to make it work. They are probably posted in various places, but ISI has them available for free on their website.
OK, I forgot that that's not quite true. From
THIS page on the official rFactor.net website you can download the editor itself, "Waypoint and camera editing tools" is what you want there. You also need a DEVFILES download to make it work and for some reason they haven't had that file included with the editor on their site at least for a long time(doubly bad since that file is the one with the instructions).
I found it hosted at
http://www.filefactory.com/file/agb595c/n/rF_AIW_Editor_DevFiles.rar as well as at
http://www.general-files.com/download/gs6217d57ahfi0/rF_AIW_Editor_DevFiles.rar.html . I just found those two along with several other potential links(most of which seem to point back to filefactory) by googling rF_AIW_Editor_DevFiles. Hopefully they work and don't have any evil stuff attached. I checked the second one(which gets it from mediafire) and it does seem to be the right file and my system says no threats detected. The first one is probably the same place I found it years ago when I needed it. That file should be always kept available somewhere since it's a rather important file, but I didn't find it after a quickie search of a couple simracing sites for whatever reason.
If there is another easily available application that works better I'd love to hear about it as the official editor is rather clunky and annoying. The instructions are passable at best(that does not mean you shouldn't read them)... I did find an expanded instructions file somewhere long ago but it wasn't a very big improvement. Tinkering is probably your best bet to learn how things work. The basic means of using it however is fairly simple since it runs as a stripped down version of rFactor with extra tools for editing.
To re-set the racing line you just tell it you want to redefine the line and then drive the course. You have to run an entire lap at a time which is a bit annoying but once you are close to what you want you can push and pull and smooth individual sections to solve minor problems. Each time you improve your time(which you can reset in the menu) it will re-write the line. I've never used it with point-to-point tracks so I'm not sure what might be different there. I imagine you'll just have to keep stopping at the end and starting over at the start line instead of running lap after lap.
It can be somewhat tricky to maximize the AI's pace since they drive and think differently than we do, but to go from a bad line or no line to a decent one is fairly simple. There are some also advanced methods that I don't understand at all(I think it has to do with the "cheat speeds") which can dramatically improve the AI's performance beyond just a decent line, but I can't help there except to say that tweaking the line after those have been setup will undo them. Most of the tracks I've gotten into haven't taken advantage of this feature however.
It does seem daunting at first and the editor is not the most pleasant program to work with, but after a little bit of tinkering the basics will become quite simple. It is still very useful to learn how to do this, because as I said a lot of tracks that are otherwise just fine will have some very simple things wrong with the AIW that make them unusable with bots or even cause problems with human drivers. Just remember to of course back everything up before messing with anything.

The AIW editor only changes the AIW file, but it's good to have baseline backups of all of a track's files if you're modifying it. The editor can be more than a bit finicky, so it's good to make very frequent backups of your modifications to the AIW as you get major steps complete.
The AIW file itself can be viewed in a text editor which can help you understand what changes you are making and what other people have done on their tracks.
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A couple of tips from my memory:
Before loading the the AIW editor, change your options to set time progression to real-time and set the time of day to broad daylight. You can't change these options in the AIW editor but they will still take effect.
Immediately find the option in the menus to disable the menu hotkeys. These will make common keys you use while moving around and looking at the track execute menu commands that you won't want to execute. They are always turned on when you start the program so you need to disable them every time and re-enable them temporarily when doing something you find them useful for.
Redoing the pit-lane's line will wipe out all of the stuff connected to it meaning you'll have to replace all of the pit stalls and all the lines for driving around in the paddock. I'm not sure if garage spots are affected or not, but it's still a ton of work to rebuild. It's too bad too, since many tracks have pit lines that don't properly join and diverge from the racing line which is what causes the AI to swerve abruptly and slow down dramatically while switching from one to the other. Unlike the racing line, there's nothing you can do(except I suppose text-editing) to adjust that line once it has been created.
A common problem is that when you tweak the racing line you will lose the rubber marks on the track that denote braking spots and turns. These are calculated from the line you set, but you'll need to run a command from the menu to do it. I think it's called "recalculate race groove" or something similar and I believe it's in "advanced options" (or something similar). In my experience it doesn't always seem to work the first few times I do it, but that is what creates the groove.
Hope that helps.