I did say I thought it was an engrampa bug.
And sure enough, when I investigated, I found the bug:
Upstream file-roller bug
Engrampa bug report
Engrampa fix
...
Uninstall p7zip and Engrampa works properly. It is flagged out-of-date and presumably a fixed version of Engrampa will appear in Arch soon.
Regardless, since Sigil does not parse the stdout of `7za l` in order to figure out how to extract a plugin using minizip, it is completely irrelevant what Engrampa does and the Engrampa bug is merely a fun coincidence doubling as a red herring.