I'm not a lawyer, but I can throw in my two cents.
I suspect that the Linux kernel is NOT an example of co-ownership in the legal sense. I think co-ownership means that multiple people JOINTLY hold the copyright to the same larger work. But that's not the case here. Copyright can only be assigned to someone else (including a group) by a written agreement, and there's no such agreement in this case. What's more, there are clear records of who contributed what, so at least in principle you can found out who contributed what (see "git blame", though obviously that is imperfect). Therefore, there is no co-ownership. So presumably the copyright for each patch continues to be held by the contributor.
So what we have is a bunch of separate works, the copyright of each is held by different people.
I think this is bolstered by Linus Torvald's statements over the years explaining why he does NOT want copyright assignments. Courts are supposed to examine the intent of the community, when there is one.
But I'm not a lawyer. I'd love to hear what a REAL lawyer would say.