The DM I had at the con was an experienced, professional DM. They leaned into what they termed the rule of cool just as much as the OP's DM. In many ways he was a fantastic DM, he just wasn't the right DM for me. So is his definition of the rule of cool also incorrect? Who gets to decide?
It’s going to vary by table and DM. Obviously you get to decide going forward that you won’t play with him any longer knowing now how he applies it.
Which is why I keep harping on the fact that we have no definition of what the rule of cool means and how it's misleading and annoying to lump everything together. There's a massive difference between allowing improvised actions not explicitly covered by the rules to ignore the rules of the game completely and let the player do what they want if it is fun for the player.
And we never will have a formal definition because it’s something that has been adopted by the community and is similar to rule zero.
The rule of cool has such broad definitions. So if the OP's experience and my experience are not rule of cool (even though the DM I had would disagree with you) what are they? Because the style isn't inherently bad, it's just that our experiences went too far on the spectrum of "follow the rules" to "always say yes" for us. We are arguing about different things, you seem to want the rule of cool to mean any and all improvised actions, the way I've seen people use the rule of cool goes far beyond that.
It’s as I said - a bad call perhaps by a bad or inexperienced DM.
I think “rule of cool” is when a player argues for an impossible or unlikely action on the grounds that it would be something cool within the greater story as it’s happening in the game. It’s an appeal to the DM to use rule zero in the player’s favor. Most of the time it should still require a roll.
I’ll give you an example of my favorite “rule of cool.” Our group was fighting a bunch of mercenaries and it was a difficult fight with one PC down, and a bad guy who was doing a good amount of damage from some cover reloading a heavy crossbow. For whatever reason, the DM said that the bandit was using a hand winch and was knocking the bolt into place. The party’s thief (yes, it was 2e) was going before him but was not in range to really attack. He asked if he could use his chime of opening to cause the bolt mechanism to release prematurely, basically causing the guy to lose a turn reloading. Should a chime of opening be able to do that? You can argue the mechanism is a lock of sorts…maybe? Definitely not what was envisioned in the description of the magic item. But it was clever, concise, and tactical and the DM decided “that’s also just cool.” Guy misfires, goes “oh crap”, tries to reload again and the party’s fighter takes him out next round.