Using leaked source code to Nouveau is illegal whatsoever, and the mere existence of the leak means that devs have to more thoroughly audit incoming contributions. The similar thing happened for ReactOS in 2006; they had to audit all the existing source code for more than a year.
I doubt the hackers are actually trying to help open source, it probably just occured to them that open sourcing would be a way to somewhat future-proof them not putting those restrictions back in for the next generation of cards
I'm not sure if they actually have the expectation that nvidia will meet their demands at all, I certainly wouldn't. Because why would nvidia trust that their word that they won't leak anything is meaningful?