I read that, but it doesn't adequately explain how Butterfree and Yanma etc. meanwhile are Bug/Flying when they easily could have different secondary typings (it's not a stretch to see them as Posion and Dragon types, respectively). Plus, Flygon's Dragon typing doesn't really account for its wings, its ability does (Not all Dragons need fly/wings, just look at Dialga etc. Then again, the anime manages to construe this by having them float!).
In the end, it's just GF's whimsical discretion really (Yanmega as Bug/Dragon would have been killer though!). Or maybe it's just the fact that having only two slots for types limits the scope of a Pokemon.
Back on topic, without anymore information it does seem likely to me at this point to be Dark/Flying. All I'm saying is that given GF's track record it could easily wind up as something else.