The numbers claim is coming from https://mmo-population.com/activity.
Which yes, it's at best an educated guess and not really meant to be taken as fact - of course, all the click-starved gaming news websites and youtubers don't really care about that, as long as they get their clicks.
With that said, I would say at the present moment it's definitely not impossible for FF to have more active players than retail or classic individually, especially since retail+classic combined still have a lot more.
Bellular I believe has outright stated that FF is at the moment a better MMO, although WoW still does some things better. With that said for a long time he seems to say more what he thinks will be spicy for the videos than what he actually thinks or makes sense, so combined with 9.1 delays I would take that with a huge grain of salt - but that's just my opinion.
Asmon I'm not sure if he has played yet, only watched videos on it (including bellular's) and his general position so far is that he doesn't particularly like the art style but overall wants it to be a good game because he believes it's best for games to have competition and to "inspire" each other.