I was surprised they weren't using Kiln [https://www.fogcreek.com/kiln/], a Fog Creek product. I know SO is independent from Fog Creek now, but still a bit surprised at it. I wonder if there was a migration off at some point.
Yep...for the on-premise reasons listed in the article. Once upon a time a lot of projects were on Mercurial, hosted by Kiln. The Stack Overflow repo specifically has always been on an internal Mercurial and then Git server. Originally this was for speed, now it's for speed and reliability/dependency reduction.
