There's no need to rope conspiracies into this. The structure of incentives in corporate environments have people in the company not looking for something they don't want to find. Managers would rather not know about violations and be able to tell their boss that their team are pros who got the job done fast, than investigate their reports for violations and quite possibly find a big mess that needs to be cleaned up.
Point is, shit that shouldn't happen routinely does happen anyway. "That wouldn't happen because it would be illegal" is generally bullshit.
Companies of this size have routine code audits that would find copyright infringements of this sort. If the employee literally brought in source code files and not even change the name of the directory like it was highlighted here, it'd have been caught. The audit is also not conducted by the manager, but a third party. There is no possible way this would fly unless the individual intentionally stripped the source of copyright notices (the individual's fault), or the source code was just sitting on their laptop and never acted upon, etc.
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).
The article mentions the directory containing the files was still named as it was in Valeo. If he didn't bother renaming the path, I'm not sure he bothered removing the copyright.
Point is, shit that shouldn't happen routinely does happen anyway. "That wouldn't happen because it would be illegal" is generally bullshit.