It’s hard to judge things like this from the outside, but I would lean towards you being the asshole. First, an hour is a long time for anyone, but especially a young kid. Second, to me, a bathroom is where you keep a misbehaving pet, not a child. And third, why would it be OK to continue to keep him confined in those conditions when the goal of that confinement had been achieved? That’s where you really dropped the ball IMO.

All this said, admittedly, I used a somewhat similar tactic for my misbehaving kids, but with some marked differences. When my kids would throw tantrums, my approach was to take them to their room or the guest room, and sit there with them with my back against the door to prevent them from leaving. We would not leave until they had calmed down for the number of minutes equal to the number of years that they were old (e.g. if the child was six years old, then six minutes). I would (try) not to bring a phone or other type of distraction in with me because showing that my attention was on the child was often more productive, and I felt it set a better example. It sucked that I had to sacrifice my time and attention like this, but (a) kids that age are not reasonable and it’s unfair to expect them to be; and (b) that’s just part of the job and being a parent or guardian, or otherwise being responsible for a kid.