Yep; too much of a niche within a niche. Not that the market isn't there, but I just can't see droves of the limited numbers of capoeristas flocking to buy this game in an effort to improve their skills. In general, the only thing you get better at by playing video game fighters, is your ability to play video fighting games. Entertainment does not = reality.
"Learn martial arts" is just the marketing lead (looks like its Superchunk's lead, not the developer's). Based on the early beta footage shown, you won't learn anything more than you would from playing Virtua Fighter, or Eddy Gordo from Tekken.
But what about motion controls and its ability to let the player pantomime and learn moves? What about them? The majority (closer to 100% actually) of Capoeira is done with the feet, legs and hips. There aren't any hand/elbow strikes in traditional Capoeira. So... I'd expect to see remote motion functions substituted for kicks, which doesn't mimic reality.
Still, people should give the developers the benefit of the doubt until the game becomes more fleshed out and/or is released.