They can "control" them in any meaningful way if they use them for access of things that you do not allow or denies access for things that you do allow. If neither are happening, then you're effectively the one controlling, not them.

The specific issue at hand is sharing. With passwords, I can easily share my passwords. Is it easy to share passkeys? And could doing so be prevented by Microsoft?

The point of passkeys is that you can have many of them unlike a single password. Each device should have its own passkey that I can revoke if my device is lost.