Just like any foss project, there some level of trust if you are going with the main distribution. In theory you are correct that not much is stopping them from releasing a malicious update, but because it is open source, soon enough people would notice that either they released new code that is malicious, or that the new version does not match the source code. That kind of scenario is known as a supply chain attack.

Since the code is open, you can literally read it for yourself to see exactly what the apk does. You can also fork it and modify it however you like, just like the creator of Molly did (Molly is a fork of the Signal client that adds some security features)