I don't know exactly which 'leaked' cars are under discussion here (if they're more GT cars I probably wouldn't be interested) but when you share non-public development builds for testing (or whatever) to your 'trusted people' are these marked as such, eg ''WIP dev version - not for public" in the UI version number? It might draw people's attention to the fact that it's a leaked incomplete WIP and might limit its distribution on some websites. Otherwise, how is Joe Public supposed to know that it's WIP or leaked? It's just anbother car to them. Just a thought, it might help, it might not. Can't do any harm though.
And of course, a 'private' discord is essentially just a corral of random people you met on the internet, so unless everyone in there is 100% trusted (and how would anyone ever know?) then it's reasonably likely that one of them might be a rogue. As RMI Wood already pointed out, things only leak if the modder shares them, ergo: he (or she) is always the original source of the leak. The mod obviously becomes a lot more desirable (and people will be more likely to share it or hunt it down) if it's been teased in screenshots on forums, FB or wherever with 'private mod' stamped all over it. Not suggesting that's what you've done, but it is a general pattern in the modding community where private brag posts are only matched in their number by PayPal links.
Anyway, I hope this hasn't been too upsetting for you and you do manage to complete the cars and release them officially to the public. It must be frustrating knowing that work is out there with your name on it but not to the quality that you originally intended. That's why I never understand it when modders go on these rants (and by God, there are an awful lot of these 'leak' rants) and say that they'll never update the car now that it's leaked... as if they're happy for the 'bad' version to remain the only example of this particular work associated with them.
And for the record, when you attack GTP by posting on GTP you're actually attacking yourself.

GTP isn't a hive mind, it's thousands of individuals each capable of individual thought.
At the end of the day all of this is just pretend cars for a video game and nothing worth raising your blood pressure over.