I think when Bob said, "We'll maybe leave in a couple of hours," that was the perfect opportunity for Alice to say, "Oh, so noon, then?" And then Bob could say either "yes" or "Well, maybe earlier if everyone is ready." But I don't think it's solely the fault of one side or the other, I think they both made a lot of assumptions about what other people were doing. I would've interpreted a "couple of hours" from 10am to be noon-ish, vaguely 11:45 to 12:15, but nothing strict. I don't think I would've made the leap to, "Whenever everyone's ready," unless these words were actually said.
But, on the other hand, as I mentioned before I would've been ready to go earlier anyway, because my experience with group outings is that miscommunications are common and I wouldn't want to be the one holding everyone up. I do think that if Alice could've gotten ready more in advance of the deadline, she should have done so, even if she was completely certain of the deadline in her own mind--she left her "getting ready" until the last time slot when she didn't have to, and that might have inadvertently held people up if she couldn't find a shoe, her hair dryer tripped a circuit, the hotel conditioner left her hair tangled, etc..
To me it seems like one of those situations where I wouldn't be able to control what anyone else was doing, but I could definitely make sure that I was ready to walk out the door whenever, so if we were ever late it wouldn't be my fault.