I think a situation like this is better handled one on one--wait for a natural break, pull the interview lead aside and explain politely that you appreciate the time and consideration but think it is just not a good fit culturally. Then the lead can explain it to everyone else and excuse them from the rest of the interview process.
Announcing any kind of surprise or unwelcome news can be problematic in a group situation. When people have an emotional response, they want to express it, and in a group situation the mutual reinforcement can quickly scale up the emotion.
This is why it's generally not a good idea to quit by standing up in a staff meeting and announcing it to everyone at once (unless you're trying to make a scene :-)). It's much smoother to tell your boss first, so that they can help manage the emotion of the team.
From what I can tell of the story, he was never left alone with 1 person at any time. He was ushered from room to room, meeting groups of people the whole time.
Personally, I think the culture sounds great. But if he didn't, there's no point in wasting his whole day there. He was polite in telling them that he didn't think he'd fit and there was no need to waste anyone's time.
Had this been a 2-hour interview, instead of a whole day, I'd say stick it out and be friendly. But not a whole day.
There's usually a lead for an interviewing process like this, and I don't think it would be that hard to say something like "hey can I have a word for a minute" as you're transitioning between groups.
Even if there is, that doesn't mean he'll know who the lead is. I have been in interview where I have gone from person to person with no one in two consecutive sections. As far as I could tell there was no one person to discuss sensitive issues with.
Never left alone with 1 person? I find that hard to believe. And who's doing the ushering? How hard would it be to ask to speak with the hiring lead or really anyone?
Announcing any kind of surprise or unwelcome news can be problematic in a group situation. When people have an emotional response, they want to express it, and in a group situation the mutual reinforcement can quickly scale up the emotion.
This is why it's generally not a good idea to quit by standing up in a staff meeting and announcing it to everyone at once (unless you're trying to make a scene :-)). It's much smoother to tell your boss first, so that they can help manage the emotion of the team.