It isn't that it will have been trained on the source code. The "decompilation" craze has it in the name... it is being done by decompiling the game and then modifying it. I don't think it's even a question as to whether or not the game companies are going to come down hard on this and I would not recommend to anyone that they jump up and volunteer to be the legal test case for that.

A clean-room implementation is going to be much, much more effort than a "decompilation". Even a decompilation is, I think, more than the hype is claiming. You can't just point at a game, say "decompile it and make it work on Windows" and get a result in one shot. It will take playtesting, cycling around, QA, and time to polish it up. But that will still itself be much easier than an actual clean room implementation. Let alone getting sued by the owner anyhow and having to prove it was clean room in a typical chaotic individual dev environment.

Well funny enough, the code for GTA5 did leak not too long ago...

It's not though. We get the machine code for programs we run on our PCs. All the logic is provided to the end user. The only protection these days, is not doing all the game logic on the end users PC.