This is an interesting discussion.
I've always been a stickler for punctuality, so I aim to be early whenever possible. When I was in middle school, one of our teachers told us "Early is on time, on time is late, late is unacceptable." What she meant, essentially, was that being "on time" for class meant being ready to begin working when class began. Students who were entering the room when the bell rang were late if they still had to take their seats and get themselves organized after class start time. It was obviously not okay to walk in after the bell rang, barring any emergency.
It sounds like Alice was aiming to be "on time" late. That is, she was aiming to be "let's start leaving" at the "leaving" time. I think that when other people are involved, it is a safe rule of thumb to try and be a bit ahead of schedule in the event of unforeseen circumstances.
I tend to see deadlines as meaning "this time at the latest" rather than "this time exactly." Take airplane departures as an example. The doors usually close about five minutes before departure, but if everyone is on board, then they will sometimes leave early. Obviously, sometimes circumstances (connections, bad traffic into the airport, getting stuck behind a pack of twenty first-time flyers who have knee-high lace-up boots in the security line, etc.) prevent people from getting on the plane ahead of time. I was once the woman sprinting breathlessly up to the gate as they paged me (twenty minutes before departure time). However, what Alice was doing was like sitting in the lounge sipping a martini for as long as possible while everyone else boarded and took their seats.
I think the bottom line is that when multiple people are involved in an excursion, it's rude to act as though you're making a solo excursion. You should pay attention to others and try to follow suit as much as possible. Even though Alice had heard "we're leaving at noon," she failed to notice that others seemed to be getting ready a bit faster than expected.
I think a lot of people would assume that a person who was fully clothed and watching TV was trying to stay out of the way and occupy time until everyone else was ready to go. In fact, the other people may have been hustling because they assumed she was waiting on them.
I guess the lesson learned from this is that Alice and the rest of the group operate under a different set of assumptions and understand nonverbal cues very differently.
(Sorry if this winds up being a duplicate; my session timed out.)