The extension describes itself as Wikimedia specific messages.
The longer version from the README.md
The WikimediaMessages extension adds Wikimedia-specific internationalisation messages and grammar, as well as Wikimedia-specific overrides for license and other legal-related messages.
It's unclear why various JS, CSS and LESS are making their way into the extension...
Maybe the scope of the extension is purposefully changing, and it's more than "Messages"; it's WMF "common" code, and renaming the extension isn't worth the effort (as we've seen in numerous other cases). But that doesn't mean the documentation in the repo, where in WikimediaMessages and at https://www.mediawiki.org/wiki/Extension:WikimediaMessages can't be updated.