I believe that in previous kernel upgrades I just had used what I think is the standard process with no error. But I had failed to upgrade to any sub-version of 5.10.
I don't remember what moderately crazy or nonsensical attempt I tried along the way, not that many, but finally I've solved it in a way I just didn't expect.
I simply backed up/renamed the source directory, and downloaded again with git, and did the two or three commands listed on the README. And it just...