You have to attempt to look at it from his prospective, which almost none of the posts I've read so far have tried to do (it's mostly people instinctively calling him a man-child or likewise, without much hesitation). Because of the success of Fez, I don't believe that Fish is really all that obligated to make more games if he doesn't want to. After all, he does't really need money from making games to get by anymore. So if he continues making Fez II it's purely out of want. He can easily go on and pursue other things.
So, if he finds himself getting verbally abused by the same people who plan to play his game and then has the gaming media take his words way out of context (like the comment he made about Japanese games being twisted to being a hate speech about Japanese culture as a whole) with the expressed purpose of making him look like a terrible person. Why the hell would he put himself through the stress of making a game (video game development is an extremely stressful job with or without people saying shit about you) if all you will get in return is news headline about how he's an asshole. having his public image being thrashed by people he doesn't know (and sure as hell really don't know him) without being able to do anything because if he retaliates, he only gets more flack.
If you had a job that you did not need to make ends meet and hated most of the people you worked with, wouldn't you quit?
If you had a girlfriend you hated and didn't really need her to be happy, wouldn't you just dump her?
It's not childish, it just makes sense, you'd be stupid not to.
If I was in his shoes, I would probably would do the same (minus the Futurama reference, solid as it was).
P.S. even though most famous developers (like most public figures) get shit thrown there way all the time, Phil (like Jon Blow) tends to have his public image be twisted to something that most likely doesn't resemble who he actually is in a very negative way more than others. People's ability to tolerate it varies. It's not like the quantity of the negativity is trivial.