No, no, I was in my teens in the '90s. I'm mostly parroting what I heard from older gamers and news outlets in the same time period, combined with a bit of admittedly challenging personal research and speculation. I'm always really surprised at what a small but dispersed population can support in terms of commerce.There's no real question '90s numbers were in the millions (single-digit, I'm sure). I don't know why you'd think they were lower than that.
Are you younger? Like weren't around in the '90s? I don't think there is any possibility a few hundred thousand people worldwide could have supported the sheer number and diversity of FLGSes, as well as other RPG outlets, and the huge and diverse RPG market, which had to exist in print - far harder to do than in PDF/PoD! - particularly as those RPGs often had vast arrays of supplements (far larger than we see today). As an example, throughout the '90s, the largest media retailer in the UK - Virgin Megastore - maintained a large and diverse RPG section in its stores. It actually got bigger and better-stocked from about 1991 to 1995, and wasn't that much smaller than the console/PC games section in that period.
There are no really good sources? But going by the latest edition of the Orr Report (4Q2021), online D&D5 games on Roll20, at least, capped at 50% of all games. I feel like if there were that big a discrepancy, it'd show up more clearly in an assessment like that. But other games (admittedly, including earlier editions of D&D) do seem to be keeping up.Also personally I very much doubt that the non-D&D RPG playing community is currently 100% the size of the D&D-playing one, right now in 2023. In 1994 or 2000? Sure. In 2010 and counting PF1 as "non-D&D" (questionable)? Definitely. Now? When D&D has jumped from 10 to 20 to 30 million players? You think non-D&D RPGs just jumped up with it? I surely do not. I think it's much more likely we're looking at 30m D&D players and 10-15m other RPG players (on a good day).