We found every Cloud SQL instance runs in a Google-owned project called "speckle-umbrella-<num>", with <num> being a number between 1 and 80.
Each speckle-umbrella-* project contains several Cloud SQL instances, of different customers, and they do seem to be on the same network and without proper firewalling, because we ran zmap on 10.0.0.0/8 and could see several IPs with the MySQL port open (We did not try to connect to any of them though).
This problem would have probably been avoided if Cloud SQL used different tenant projects per customer (Something most other GCP services do), but for some reason it doesn't do that.