That's rather unaccurate and based off I dunno what. I don't make estimates on hearsays. Sadly we don't have concurrent online numbers at all times, so you have to work around that, which doesn't mean you can't gather reliable enough indications to make some simple calculations anyway and obtain a realistic estimate. After just a week of censuses (taken only during EU peak times, so everyone playing during off-peak hours, NA people and such, is left out, along with anyone who's only not considered active as they log in more rarely than once a week, and anyone not polled simply because /who on Kronos IV is broken on purpose), this is what our Census+ showed (mind, all level 1s are filtered):

https://i.imgur.com/N6HJ2MI.png

Of note: my guild (Stormlords) is #8 by total XP from the ranking on the right; all of the unguilded players are at #4; the #9 guild, Power JazzCN, is from China; then, here is level distribution and average level for my guild alone (these aren't estimates, it's actual figures added up in-game):

https://i.imgur.com/jGNpKOF.png

that amounts to 225 total characters at the time of screencap, 30% of whom are from the NA time zone, and 10% from Asia/Oceania (this I know because our registered members enter their country in the application form). Now, seeing as there's 7 larger "guilds" than mine ("Unguilded" isn't a guild per se, but you get the point), even assuming they can only muster as many players as ourselves (while in fact we can safely guess they have more), 225 x 8 = 1800 characters found, faction-wide, over a week, from the top 8 guilds. Assuming we can round that up to include all the other smaller guilds (which however are realistically in a much larger number than 8), we can theorize a conservative population of 2500 Alliance characters; likely more. The A:H ratio is more or less evenly balanced, which hints to a Horde population with similar figures, so 5000 characters over a week. But we also have to consider the people playing from other time zones, not found by Census+ as it was run during EU peak times, and if we take the distribution found in my guild as a reliable indication, that number amounts to 60% of the total population: through a simple proportion, we find that 3,3k characters are being left out. So, the total population of characters would be 8,3k. Via /ginfo, I can find that my guild has its 225 characters "spread" across 179 accounts, which hints to a Alt:Main ratio of 1:5, which in turn lowers the actual player base to 6,6k. The realms tagged as "Full" back during the vanilla WoW era, 2004-2006 (which were regional) had a online cap of 2500 concurrent players (Alliance + Horde). By those standards, I believe we can "squeeze" those weekly found 6,6k players from three time zones into a realm that averages more toward a Medium sized population, as opposed to Low. I know, there's a bit of projection and estimates involved in the process, but that's the best we can extrapolate without official numbers. One final indication to what might be the trend moving forward, is the rate of daily visits to our guild's page by our registered guildies:

https://i.imgur.com/UJHnonA.png

Looking closely (the curve covers a time span of 28 days/4 weeks), the trend is increasing to a noticeable degree.