It is certainly unkind, when a developer asserts the opposite of what you have assumed about their code, to double down and imply they are lying.
> the original commit being 4-5 lines long (what did claude do then?)
I've said "rebase onto <newbase>" and let it handle all the merge conflicts. I wouldn't expect this particular commit to conflict with anything, but it could have been part of a big series where it'd be worth doing that instead of running the rebase command yourself. It wouldn't surprise me if I picked up some Co-Authored-By:s along the way.
I hope if this doesn't come across as unkind towards the dev who gives their time and energy to the project. Grateful for that.