Just as I stated in my post, yes; the SP progress unlocks the cars for free as separate MP challenges.
And given that, as also stated, no car purchases, no car mods, no budget, and even no garage progress is carried from one mode to the other, they are separate garages. You cannot, as further stated within the review, tune up a car in SP and use it in MP; you have to do it all over again.
To add to this, and to somewhat offer a quick tip to others, this more-or-less is also the case for rewards gained for completing activities and finding collectibles. The cars that you get when you complete these challenges are added to both your SP and MP garages, but the cash earned for beating each activity/finding each collectible, including
the money you get when completing the sub-challenges for each of task, is sent only to the bank in whichever mode you completed the challenge in.
For those who think that money is scarce in SP, Multiplayer, at least in its current state, is a lot less forgiving in that regard, largely because the money earned vs. time spent is significantly worse. To that end, it might be worth doing at least the collectibles in MP, or trying to save up enough cash to build out a S+ car that's capable of smashing all the Speed trap and Jump records.
Guys would you say the physics are closer to Forza horizon this time around?
I'd say that the physics in Unbound are the best overall physics that we've seen in an NFS game in a long time, and I'm enjoying it quite a bit personally, but definitely doesn't come close to Horizons balance of arcadiness and simulation within its physics. There are some annoying quirks as mentioned in the review, like how cars (especially certain S+ cars) having bouts of random snap-oversteer, and how drifting, and building a drift-focused cars in particular, is incredibly lackluster.
IMO, the game is still quite enjoyable, but it's very
far removed from FH4/5 in terms of physics.