@DiapDealer: Yeah, I had forgotten how awful (by 'awful' I mean 'missing a single parent directory', preferably following debian's formatting of project-version ^^) the directory structure of Sigil's source packages were... And thus realized that the Mint instructions aren't very clear at all, there's no 'sigil' folder anywhere in Sigil's source zipballs...
So I was just assuming the sigil directory in question was manually created and where the zipball was unpacked, meaning it's building in the toplevel source directory

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