Of course, a cleanroom procedure is a given in this case. History has shown that it is entirely possible though - see ReactOS for one very public example.
But also, not everyone cares. See game modders who routinely use source code leaks to make game patches. I'm not talking about big companies that want nvidia would want to sue, but individuals and small groups, who are not a big enough target and would likely remain anonymous anyways.
That thread goes on to describe several very plausible reasons that a clean room implementation could have resulted in all of the coincidences the commenter pointed out.
But also, not everyone cares. See game modders who routinely use source code leaks to make game patches. I'm not talking about big companies that want nvidia would want to sue, but individuals and small groups, who are not a big enough target and would likely remain anonymous anyways.