Neither, uses libgm from mautrix-gmessages (go.mod). That library speaks the protocol Messages for web uses, Google's relay. Google doesn't publish an official Messages API, so this is an unofficial client, it could break if google changed anything.

So far it works great, the repo is private right now because I was considering turning it into a cheap $5.00 a month site for political spam blocking for Android. But considering it uses libgm, I'd have to rebuild it from scratch in order to do that, so now it's just for me...