I was under the impression that (perhaps from the above and/or maybe from reading something else too) the team got to the clue room and didn't get any riddles right. I'm guessing that either because the production team didn't have a "wall monster eating dungeoneer" effect or that the next team wasn't ready yet, they decided to let the team live on. So, they went back and filmed the giant scene later on in which he gives them the TRUTH spell, and they also re-film the necessary parts of the clue room scene.This quest was unusual as it was the only quest in Series 2 that did not start with the Wheel of Fate. It is believed that this team may have had their quest restarted to enable them to get the TRUTH spell from the Troll and guarantee themselves at least one correct answer in the clue room, though this is unconfirmed.
At the same time it would have been a lot more subtle of them to re-film the clue room scene with either a different riddle or having told the team the correct answer, if they were going to let the team live anyway.
Maybe they opted for the TRUTH spell scenario instead as it was interesting and different to anything that had been done before.