https://en.wikipedia.org/wiki/Firebase
Firebase is a platform developed by Google for creating mobile and web applications. It was originally an independent company founded in 2011. In 2014, Google acquired the platform and it is now their flagship offering for app development. History. Firebase evolved from Envolve, a prior startup founded by James Tamplin and Andrew Lee in 2011.
Google Cloud + Firebase. Firebase projects are backed by Google Cloud, letting you scale your app to billions of users. Read more Trusted by the largest apps and games. Development teams around the world—including NPR, Halfbrick, Duolingo, and Venmo—use Firebase to ship their apps.
Please read carefully.
Your assumption ties Firebase with Google analytics tracking, which are clearly not the same thing atleast as far as this post is concerned, or the Signal app and how it works. The assumption further entrenches into the territory that Signal is an anonymous app (which it does not claim), and Google Firebase is somehow snooping on messages and metadata, which is untrue (they claim their app is only private and secure, not anonymous).
Glad that I scrolled down and found a proper answer :) You're right. Google might track people using Signal this way, but it doesn't mean they have access to information they don't already have.
It takes so much time to detangle these narratives pushed around, people thinking or in some cases claiming they have no idea. One wants to criticise, there is plenty legitimate stuff, but do not be a propagandist for the sakes of wanting to criticise.
It hurts everyone in the end.