As you say, there are no tell-tale signs of calibre in this epub. I don't know Sigil well enough to know what to look for. It may just be that the original epub creator thought the TOC was fine because the epub's TOC
is fine when you open it in the calibre Viewer or Firefox epubreader. It's only the conversion process which has highlighted the problem.
I guess it will remain a mystery.
I think you'd better remove the attachment before the forum police descend.