And based on this code from calibre:
https://github.com/kovidgoyal/calibr...sh/kepubify.py
There are quite a large number of quirks being accounted for including wrapping the contents of each body tag with both outer and inner divs, wrapping sentences in spans, etc, that really do not make reading and writing kepub files in Sigil directly possible.
So if this is something you plan to do, you should either convert the .kepub to normal epub3 using Calibre or write your own input and output plugins for Sigil probably using calibre code as a model for all the changes needed. Then make sure your css does not use tag selectors for div and span, since they are being injected for use by kobo.
So although you can read in a kepub into Sigil, it will be full of such junk that most people will want to clean up or remove, to actually get a clean epub3.