<tag></tag> is perfectly well formed XML. I suggest you tell your client to read the XML specifications.
You don't need Linux to do a build, indeed you don't need to do a build at all. calibre can run from source without needing building see
https://manual.calibre-ebook.com/develop.html
I am not going to accept this change in calibre as it has the potential to break things for people for no actual benefit to any calibre user.