Think about the large scale conspiracy required to keep something like that secret from an entire company.
Point is, shit that shouldn't happen routinely does happen anyway. "That wouldn't happen because it would be illegal" is generally bullshit.
So I think the question (and I'm not a lawyer...) is whether Nvidia conducted such audit and to what extent (if any) the other company's source code was merged into theirs (versus just sitting on the laptop).
If he would have been any smarter, he would have kept the stolen files on a private computer and he would have gotten away with it.
Why do you think that either the ethics or the willingness to risk breaking the law would be different between people being hired by a company vs. people working at a company?
A company's decisions are the decisions made by people who work there, and who once were being hired there. For a company to be complicit in encouraging a new hire bringing illegal IP with them requires one or more people at the company to make the same ethical/risk decision as the person joining. It doesn't require an announcement at an all-hands followed by a long slack conversation with the legal department, it doesn't need to be a huge scale conspiracy.
And it's no harder for a person who said "hey come bring your old employer's IP to my team" to keep it a secret from the entire company than it is for the person who joined doing that to do so.
In fact it's a bit easier, as long as they weren't stupid enough to discuss in a way that gets logged, the person or people encouraging it from inside the company are less likely to get caught, as a comparison of IP may be enough to prove wrongdoing on the part of the person who brought it, while anyone in the company who encouraged it couldn't be caught unless somebody ratted them out.