There's always going to be a manager with access to keys. They get the HVAC tech on the phone and set the controls as instructed. They're still vulnerable to somebody scamming them into setting the controls wrong, but they at least have a chance of recognizing who they're talking to, and recognizing obviously crazy instructions (e.g. "turn the thermostat up to 50 degrees"). And even if they don't, the attack is a lot harder to scale with a human in the loop.