Signal is a small organization and nothing like Google. Signal is a non-profit and their code is open source.
Again, I don't trust promises anymore. Eventually they're going to need more money and that money will come with strings attached. If some org like Matrix would get bought people would just decouple themselves from their network. With Signal we can't do that because they are the only operator.
There are third-party forks and clients. Parts of the US government use one.
https://news.ycombinator.com/item?id=49678919
Also, you can audit the code.