This is one of the points he is making. It might not be legal, but it is certainly allowed with precedents being set in courts and policies being turned into law by politicians. He seems quite explicit in stating that what he is doing now with multiple 90s era games will be what happens with this years games next year, and IP protections are getting eroded at such a rate that even the big budget games get cloned with impunity. One of the quotes is about Rockstar's defense being a brand fortress and money.

Up to this point, it has never been actively stopped. Except for Take Two with the PS2 GTA games, but that was a cease & desist, so it's not determined as "illegal" either. I think that it's legal as long as you're not decrypting stuff, but my understanding may be limited.

Might be against the contract, but that’s not illegal.

If they can argue its copyright infringement, then you can be sued for it. Sometimes you can get away with just a rebuild of the engine without the assets (do not try to copy the assets!) but even then its up to the company's lawyers if they want to push it. Certainly if you try to sell your remake things get spiky. Can look at all the legal disclaimers on things like OpenSAGE as an example. But plenty of companies are cool, as long as they see no competition to their own copyright: https://en.wikipedia.org/wiki/List_of_game_engine_recreation...