Because contributions to Linux are meticulously attributed to, and remain property of, their authors, those authors bear ultimate responsibility. If Fred Foobar sends patches to the kernel that, as it turns out, contain copyrighted code, then provided upstream maintainers did reasonable due diligence the court will go after Fred Foobar for damages, and quite likely demand that the kernel organization no longer distribute copies of the kernel with Fred's code in it.

Anyone distributing infringing material can be liable, and it’s unlikely that this technicality will actually would shield anyone.

Anyone who thinks they have a strong infringement case isn’t going to stop at the guy who authored the code, they’re going to go after anyone with deep pockets with a good chance of winning.