The term is insufficiently defined and has overloaded meaning.

In the original sense of the word, i.e. creative use of technology? Debatable, and most techies would probably land on the "no" side of the debate.

Is it illegal? Depends on the jurisdiction and is up to a judge, but it probably falls under unauthorized modification of computer systems, so here the answer is probably "yes". IANAL, but lack of authorization mechanism is probably not a valid defense. AFAIK lawyers and judges do not follow the law to the letter independent of real world experience, so you can probably argue that if the UI didn't provide a way to cancel other people's appointments, then any reasonable person should realize it wasn't intended for public use and thus refrain from doing it. (Corollary: LLMs are not reasonable persons.) Just like an unlocked house door isn't an invitation to come in, you have to assume it was done accidentally (may depend on customs of the location, but where I come from it's pretty obvious).

Is it morally acceptable? Pretty clearly no. Even if it wasn't illegal, it's a jerk move.