I wonder how that 10% would compare to the fraction of WFB players in 2015 prior to its discontinuation who had started within the last 4 years. I doubt it'd be much more. Or, for that matter, how the absolute number of T9A players who started within the last 4 years compares to the absolute number of WFB players during the first 4 years of the existence of said game.
Old WFB players are a large demographic of natural converts to T9A, it's entirely to be expected that they'd make up the great majority of the player base for years, regardless of the inherent appeal and marketability of T9A as a game. Heck, I'd be surprised if the numbers were actually that much different for Kings of War or, heck, Warmahordes even. Games Workshop games are simply the gateway to miniature gaming for a very large fraction of everyone who has ever played any.