microG still heavily uses closed source Google services and even downloads executables run with the full privileges of microG. It downloads and runs droidguard with the privileges it has, giving that full access to the privileged access granted to microG.

Each app using Google Play still has the closed source Google Play SDK and libraries built into it in practice. It's extraordinarily rare to replace those libraries with the incomplete and buggy reimplementations from microG. Very close to zero otherwise open source apps are using those. The closed source Google Play SDK and libraries are therefore still part of the loop and have access to what the app can access. Many of the code paths in the Google libraries directly use Google services without requiring Play services including the full versions of the Ads and Analytics libraries.

The vast majority of data obtained by Play services is via the OS integration and permissions. Apps using it for push messaging, logging into accounts and many other features will send the same data to Google services regardless of whether you use it via sandboxed Google Play or microG.

GrapheneOS stubs out and reimplements APIs on the client side. That includes providing a complete implementation of the Google Play services location API not using on Google Play. These features are being expanded over time. They'll also be made available without having Google Play.

The GrapheneOS approach avoids giving an additional global service access to sensitive data from the OS and apps. microG has repeatedly had vulnerabilities leaking sensitive data to apps including location data. The approach of having an extra service sitting between apps and the OS which handles sensitive data means having to add all the same privacy and security checks as Play services which microG doesn't do in practice and it has caused severe privacy leaks. The main location leakage was eventually fixed but it persisted for years and there are still holes in the location service causing privacy issues.

Unlike microG, sandboxed Google Play doesn't get privileged access. That includes the executables (droidguard) downloaded by microG not getting privileged access. GrapheneOS also doesn't give privileged access to other apps such as Android Auto when those are installed as all the main operating systems using microG are doing. That privileged access gives access to highly sensitive data and doesn't require apps explicitly sending data to Google services, which will nearly all still go to Google services with a partial replacement of the middleware on the device.