Theres so many ways to check for that that don’t require decompiling the app.

You can straight compare the downloaded binary with a locally compiled binary to see if they match.

You can check the hash of app. Changing some lines of code and getting the same hash is so unlikely to be effectively impossible.

If for some reason Signal decided to do what you claim, it’d destroy thier credibility, be caught almost immediately, and only work once before the whole project gets forked, and would be true of any alternative.