The server running Signal’s server-side code.

What could a client detect? Signal is a US company and will comply with the government. The server can’t be trusted.

Some do, and that’s the point: if there’s an attempt at tampering, interested security researchers can detect it.

They can detect if a different app was installed from the store on their phone. That’s not useful for anybody to know if their own app is unaltered. Only people of interest will receive a manipulated client. So there is no security in knowing that some people received the original app.

Besides, Google runs the OS. They can change the app at runtime.