A better definition of niche in this context, as an adjective: "denoting or relating to products, services, or interests that appeal to a small, specialized section of the population".
This is a niche game. The target market is the cross section of sports fans, stat/management geeks, and sim gamers. It's not analogous to Fantasy Football, or to live action games, because there is no game to watch. They're looking for the guys who read about, and maybe obsessively follow, but don't necessarily watch sports. The guys who would rather be general manager than QB or coach.
That's not to say there is no room for growth. Of course there is. But it's got a ceiling, and that ceiling is a tiny fraction of fantasy football or EASports games on Xbox.
I'm not on FOXSports' board (and neither is Benis, I'm pretty certain) so I don't know where they believe their niche target market size and room for growth is. I do know if they swallowed the line that the previous owners fed everyone in that dev chat, about how advertising was their biggest obstacle to growth, they were suckers.
The Wire is the best TV series in history, everyone on the internet says so. But not many people watched it. Weird, right? because people seemed to like CSI well enough. So probably the writers and producers of The Wire should have made their show more like CSI. So more people would have watched it. Right.