This is from the same guy who did the previous active player estimates.
From the OF, on the methodology: "Active is determined to be any CHARACTER (as it is impossible to determine account association at this time) above level 10 that has had any change at all to their lodestone profile between the previous unofficial census and either of the 2 passes through the PUBLICLY available lodestone data. This includes any new minions , mounts, a change of job, any gain of levels, any new achievments, or even a single change in gear. So in order to be listed as inactive you need to have not done anything at all since April (the last time they did this) making it a fairly accuracte gauge for the number of active characters. Keep in mind that this means that if someone were to have multiple characters that they are active on this would count as more than 1 therefor the active subs are less than the total of active characters (though I would say probably less than 10% of people have played actively on 2+ chracters so it should be within negligible amounts)"
So people who play more than one character per account skew the player numbers slightly, but so do players who have their Lodestone data set to private (which is probably an even smaller percentage).
My reaction? Roughly 500k players at this point in 2.x's lifespan is about what I'd expect, and it seems similar to where we were at with 3.x approaching last year, though probably down slightly. But this really stamps out the doomsday prophets who were claiming the game only has like 200k players. Turns out, the game is actually pretty healthy, which shouldn't really surprise anyone.