Although the license, under section 5, states “a) The work must carry prominent notices stating that you modified it, and giving a relevant date.” One could maybe argue this wasn't done for the second change

In this case there was no modification as the code was copied verbatim.

The Piefed author should merge all that code right back in, but use git format-patch for both commits so Git authorship is preserved.

Instead of doing that, I'd just fetch their commits and merge them directly. That has the added benefit of not generating new commits with new hashes (making it easier on the fork's side). If the original commits were signed, it would also preserve those signatures.

Neither project has copyright notices in preamble comments at the top of files, or an AUTHORS.txt, or any other kind of copyright notice. Therefore there is no copyright notice to preserve as stipulated by the AGPL.

Agreed on this part though. Although the license, under section 5, states “a) The work must carry prominent notices stating that you modified it, and giving a relevant date.” One could maybe argue this wasn't done for the second change, but if that were to be merged in properly with git attribution intact (which also includes timestamps), that should also be fine.

I had a browse of what they’d done and saw something cool that I wanted PieFed to have too

This is definitely in line with the free software philosophy. It should be possible to include others' improvements to your project, if you've already got their modified source code. And “stealing” code is not even a thing (it's copyright infringement at worst, never theft).

It's also debatable whether the stylesheets copied here are even substantial enough works to qualify for copyright to begin with, but that's another discussion.

The Piefed author should merge all that code right back in, but use git format-patch for both commits so Git authorship is preserved. If the fork fellas don't like that, they should shutter their fork and go fork a project with a more permissive license that allows them to relicense to closed source.

edit: I imagine the only reason the Pylova fork authors even cared or noticed is that the stylesheet commit showed up as a merge conflict when they were merging in upstream code -- something that would not have happened if the code was merged in using a proper patch file. My guess is that there was a game of broken telephone at play here when the Piefed author heard about this from some third party.