Yup, read it yesterday. Simon Parkin is my spirit animal (he actually gave me some great advice on a piece I wrote a while ago that made it awesome).
As for playtesting... I mean, if I understood it, Bioshock's iconic reveal of Rapture underneath the sea came from that session, so that's good. At the same time, playtesting often seems to rob a game of things that would make it more interesting. I think, if I were in the position to do it (and if I find some way to actually spend real quality time on this game I'm designing, I will be), I'd use playtesting to see if I've failed at communicating information to the player.
Not "oh, I don't like playing as a girl" complaints (hi, NOLF), but "wait, we're UNDERWATER?" Basically, I want to know if people understand info required to actually understand the game, like "YES YOU ARE UNDERWATER."
It's always a case of moderation and appreciation... sure, playtesting can yield interesting results, but I still think it should be limited to the strict minimum. Even in your example, it might lead to you changing a subtle, engaging scene into some painfully obvious exposition. It's so subjective... The only playtesting I'd fully trust is my own, and even then I'd apply a healthy dose of suspicion
The BioShock story if anyone's interested: http://www.eurogamer.net/articles/2014-04-17-the-true-story-of-bioshock
It's by Simon Parkin, so, yeah, nuff said.