Warrants generally have to be for something. It’s possible that the police officers showed up with a warrant for user names and addresses, etc, and Mullvad was

Warrants generally have to be for something. It’s possible that the police officers showed up with a warrant for user names and addresses, etc, and Mullvad was able to demonstrate that they don’t possess any data responsive to the warrant. At that point there may not have been anything to (lawfully) carry away. It doesn’t necessarily mean that the cops won’t show up tomorrow with a more generic warrant, and Mullvad may have to hand over whatever information it does possess.