My experience: in 35 years of shooting black powder I'm not aware of any incidents in which a revolver ignited a charge being loaded into a chamber, whether or not the charging was done directly from a flask. This includes any anecdotal experience. For comparison, I have witnessed one incident of charging a long rifle directly from a flask result in the flask being ignited, and I've read about a small number of similar events, all involving long rifles.
However, I do not take that experience to mean it cannot happen with a revolver. I believe it would be a very, very unusual incident, though I can't reasonably assign any comparative probability.
My own personal philosophy is to avoid any possibility of an inadvertent light off if I can do so, within reason. In this case it's a simple matter to always use a powder measure or speed loader to charge any black powder gun, long rifle, pistol or revolver. I think there's simply no reason not to. I would also not permit the practice on any range under my responsibility as a range safety officer (this is consistent with the NMLRA rules).
Having said that, I do not criticize Fingers (or anyone else who chooses differently on this issue). He's a very knowledgeable and respected practitioner. and there are a lot worse practices out there.