Calling something number one implies that there must be at least a number two. Though I do not agree with the grandparent post's usage of "fool", it does seem to be the case that the Apple iPhone does outsell the most popular dedicated handheld game console, the Nintendo 3DS. However, since the iPhone is not a dedicated system, it's harder to compare. He did not claim that they were the only ones in town. If Apple can convince enough developers that tvOS is close enough to MacOSX/iOS to make develop for it, well, the reasons to develop for elsewhere do dwinde
As for why the 3DS and the PSVita are still around in an age where most people simply use their iOS or Android device for mobile gaming, well, the answer seems to be a combination of exclusive titles like Pokemon and the fact that a dedicated gaming thingy is still cheaper than, say, an iPod Touch. Oh, and the accent on offline play.