How does it actively avoid anti-mass surveillance methods? There are no metadata obfuscation mechanisms that work at scale against all adversaries. Using GCM is without a doubt a big win for privacy because adversaries that have backbone taps (like your friendly local intelligence agency) just see SSL to google.com which tells them nothing of any use, not even that you use Signal. A big encrypted packet from google.com might be a Signal message or it might be an email or a Play Store update or whatever.

People always overlook the fact that the world is full of adversaries and 99% of them can't get access to Google's datacenters. Those adversaries matter too. Look at Turkey!