Startups aren't about language diversity for the sake of diversity, they're about unfair leverage. In this case the author made a choice that didn't add any leverage to Foursquare. If anything, the author admits he boxed the company into a corner because of a rookie scaling decision.
In contrast, the Asana guys are writing their own language+framework and if it delivers on it's early promise they'll be outgunning the competition with a 10x improvement on development time and code size.
My criticism wasn't of your comment that Lift was a poor choice because it doesn't scale well. It was of your comments that the technology shouldn't be used because there aren't that many developers to hire that know/are willing to learn scala/lift or that it isn't as mature as some other technologies.
As for your argument on scalability, while I have dabbled in Lift, I am too ignorant to know how much work it would take to scale Lift versus alternative technologies. I find it hard to believe that choosing Lift is really going to make it that hard to scale. David Pollak and the creators of Lift seem like they are pretty smart people to me. However, I have no evidence to prove you wrong and therefore did not even address that point.
In contrast, the Asana guys are writing their own language+framework and if it delivers on it's early promise they'll be outgunning the competition with a 10x improvement on development time and code size.
http://asana.com/luna
That's the sort of advantage Paul Graham had by using Lisp in ViaWeb vs. his competition that was mostly writing c++.