https://lwn.net/Articles/912355/
I think stripping copyright notices is only appropriate if one entity unambiguously owns the copyright of all the code (which may imply not accepting outside contributions or requiring signing a CLA, and not copying code over from other code bases even when the license matches).
> In the process I stripped out the explicit license mention in those files, because our whole repository is licensed under AGPL3, and it goes against our style to add unnecessary licenses to each file.
It's not a matter of style. Also AGPL3 requires attribution, let alone retaining copyright information, like many other FOSS licenses.
Starting the discussion with the DMCA process is certainly a dick move, but we will never know if the other party started with it, or brought up the issue informally first.