This current announcement comes at the end of a long listing of all the projects for his company that he's working on and releasing for the rest of the year. If at the end of all that he just said "Then I'll be doing something else." and left it at that how would the response he'd get be any different? People would still be speculating about what it might be, since it would have to mean something that he didn't say anything about the project after being so detailed about everything else. It would only get worse as time went by and there continued to be no new information on what he's working on despite his being so prolific this year. The borderline obsessive compulsive behaviour among gamers would still manifest itself, and they'd probably still find ways to get irritated with him about it.
He makes himself very available to fans, as a result they're going to be wondering and asking about what he's up to far more than other game designers who don't. In his situation, what's the response going to be if he just starts saying nothing at certain points? Would that really end up being satisfactory? Or fair for those of us who don't make it a big problem for themselves?
As far as I can tell, the only way for him to have gotten out of this hole people have dug for him is to have made up some completely uninteresting projects and then stuck them into the schedule as a cover. But after the real one was announced, he'd then have people combing his upcoming projects and starting threads speculating on whether some of them are fake. Then some would get absolutely convinced that some of them were, and would complain about his "constant" use of the trick.
I think he was probably just trying to share his enthusiasm for the project with people, and that's part of what makes his website such a nice place to visit. There are a lot of other things inside and out of gaming in the next year to get excited about. Perhaps people should just choose to focus on those rather than fixating on what he might be doing if they're going to cause themselves such trouble by doing so.