Keep in mind no one wants a panic. If the issue was serious but not immediate danger it makes sense for the pilot to ask everyone to get off the plane due to 'comfort' as it is more likely not to incite a panic and any kind of emergency evacuation carrys a high risk of injured passengers from the panic.
Whatever happened might have only had the potential to turn into an emergency in the air or it might have been a slowly developing issue that might turn the environment into a danger once x happens.
I trust the pilot to know more about technical failure - even of the HVAC than the FA.
And while a technical error situation might frustrate me and might inconvenience me I'd rather the pilot takes a cautious approach and not fly with a plane with problems.

The most likely way to figure out how serious the situation was from our position would probably be figuring out if and when that flight actually left the gate and if it was still the same plane. A longish delay might mean mechanics fixing the issue (and depending on the issue passengers might not be allowed to be on the plane during the repair) a full cancelation or change to another plane will also vindicate the pilots decision.
I get the OP was inconvenienced AMD rebooked but I wouldn't blame the captain without knowing a lot more about at least the fate of the original flight