The point I was trying to make is that Sigil cannot convert book formats, and trying to compare the Sigil
editor to the calibre
converter is an apples-to-oranges comparison.
All Sigil can do is import html as a new book, and calibre's Edit Book can do that too. calibre will NOT convert when you do that, so the classes remain whole.
(Also, slightly

, calibre editor can "import DOCX as a new book" and will convert without rewriting styles. Very nice.)
If you prefer Sigil because of the featureset and way of doing things,

. If you are avoiding calibre editor because you're afraid it will munge the styles, that's a shame, because it doesn't.