I think I'd have done the same, he sounds like he did it pretty politely rather than just getting up and doing a runner. An all day interview where you've decided half way in it just won't work is not just a waste of your time, but a waste of a companies.

However, the way the room reacted doesn't surprise me. Companies like to create loyalty which can occasionally turn a bit wolf-pack. Still, a shake of the hand and a 'thanks for coming, sorry it didn't work' is always classy and always a good idea.