I couldnt find what is the correct definition of “reimplementation” but we can assume it either means “taking the binaries and bundling them in a different bundle” or “writing different code to do the same thing”.
The whole point of a sandbox
What sandbox? Not the Android app sandbox, as microG (when I used it) needed to be installed as system app i.e. flashed to the system partition.
microG may isolate the binaries or whatever code it runs in some way, but not via the Android App sandbox.
Now GrapheneOS uses a privileged app that channels the calls of the unprivileged to the OS. This is also possible for microG, so it can run unprivileged too. DivestOS does that.
The concept of signature spoofing and more is poorly pretty flawed.
I would really like if a fully open source rewrite of the core services could just work, but these apps are written for Google, contain the official proprietary code anyways, and signature spoofing only works if you dont use many hardware security features.
GrapheneOS can be extremely secure when degoogled, but it cannot securely fake to be a Google Android. And neither can microG Android.