Can they even track pirated installs ?

Firewall your pirated games

That was my thought as well, since they count installs and not use count of bought copies directly from a platform.

What if people create cracks for legit purchased games, e.g. on Steam, which only removes the Unity tracking part?

A simple Firewall rule which "fixes it" for all games installed on a machine might work as well?

I believe it might be similar or the same procedure for every game using Unity. We might see this popping up at some point.