They aren't. I use Signal as my default messaging application so I get messages from people who don't know what signal is, and I'm fine with that. If they later install signal, I do not need them to know that I was already using it.

To be honest, it doesn't really bother bother me, but just because I communicate with someone does not mean that I want to share any information beyond the contents of my communications. We might both be Signal users, but you're not my friend. If you were, I would already know it.