HE POSTED A LINK! OH MY GOD! CATS AND DOGS LIVING TOGETHER MASS HYSTERIA!

Interesting that the announcement came so far out. Thanks for backing up the claim, though it does call another into question, your asseriton that games never grow in subscribers past the initial launch. I'm uncertain why they would make a press release touting their number of subscribers six years out if they had more prior. The announcement made earlier in the game's lifespan had a similar number of accounts rather than subscribers, which suggests to me that the game had to have peaked somewhere between the early announcement that was on the official site and that 2009 press release.
That said I'm going off memory here since the pre-2005 news is no longer available on the official site, so I will hold back from asserting it as a fact.

This doesn't address your 300k subscribers claim (or current players guess for XI) for DQX but it is a welcome thing nonetheless. If you would mind indicating where you got that number, that would be very much appriciated.