There's not enough unique IP addresses to distinguish Signal servers, if you don't explicitly set up static IP addresses you're going to share an IP pool
https://docs.aws.amazon.com/vpc/latest/userguide/aws-ip-ranges.html#aws-ip-download
Sure they could tap into AWS (but it would be even easier to try to get data from Google Play Store on who has it installed).
Signal has native support for proxying via Tor in that case.