The threat model is that google and cell phone networks are untrusted adversaries. So your same concern can be said about Signal messages going through Google servers under the surveillance of the NSA. Either way the adversary sees a bunch of suspicious encrypted messages, but can't read them. But if the user has proprietary google code installed on their phone (i.e. Google Play Services), then the backdoor which this blog is warning about is the fact that now your phone software is/could be potentially compromised.