microG still uses Google services for accounts, push messaging and many other features.

microG itself has functionality requiring downloading and running Google executables as part of itself. It doesn't change the fact that apps depending on Google Play are using Google Play libraries often making connections on their own without Play services.

GrapheneOS sandboxed Google Play compatibility layer provides far broader app compatibility while giving strictly less access to Google Play code. Sandboxed Google Play runs as a set of regular apps with no special access or privileges. It's the same app sandbox the apps using it run in with the Google Play SDK and libraries built into them. GrapheneOS improves the app sandbox and permission model substantially, which applies to sandboxed Google Play in the same way.

GrapheneOS implements functionality such as location services via the OS and reroutes apps using Google Play APIs to the OS APIs. We have our own network location and geocoding implementations in the OS. We're building our own fully local text-to-speech and speech-to-text services right now.