It cannot freeze an enemy Hidden Tesla before it has been revealed.
I believe this is wrong, or at least misleading. From looking at replays, it seems like when you freeze a tesla that is still in the ground, it will still pop up when your troops are close. But, it will still be frozen for the duration of the spell and unable to attack. I would describe this exactly as freezing the tesla before it has been revealed.
Perhaps this changed in an update and just needs to be corrected now?