Sounds more like some sort of cross-compiler or converter. Which makes much more sense than an emulator as they have access to the original Flash code. They are doing it a game at a time, which suggests that it's not automatic but requires some work on each game.
It's possible that they have a solution which only makes sense for them. Nitrome are unique in Flash gaming in producing a large number of excellent games, finding a formula for them early on which they've stuck to down the years. The games seem to be built in a similar way, based on a similar framework. If so, once they port one game to HTML 5 other games can be done very quickly.