i don't use sigil.
i guess that guy from the forums just wanted to know your libxml2 version - i have the same:
find -name 'libxml2*'
i use xmllint in my weather conky
and it works, and has worked for a while.
that forum poster is probably right to point out how qt links to libxml, but i have no ideas of the implications.
- open a terminal, type "xml" and press Tab twice.
- try a different version of sigil? at least on archlinux, i have the choice between the git version (AUR) and the community version.
- check out issues (both open and solved) on sigil's github pages