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.
That's the sort of advantage Paul Graham had by using Lisp in ViaWeb vs. his competition that was mostly writing c++.