I just tried it on my phone and it seems to be working. Very cool that it works on GOS.
To verify it's off, check Settings > Apps > Sandboxed Google Play > Google Settings > COVID-19 Exposure Notifications
If you don't trust that, want more reassurance, or are required to keep the app installed for some reason, there are two other settings you can change to make sure Google Play doesn't have the appropriate access. One way would be to keep Bluetooth off. Another way would be to revoke Google Play Services' "Nearby devices" permission and that way Bluetooth can be left on safely.