Because Wii U owners never own any other gaming devices? Also, how exactly is it 'defining a new genre'?
He said "defines a new genre within the shooter genre." which is more like "defining a subgenre" than "defining a new genre" like you put it. So you're misquoting, either because you didn't understand or because you feel the need to do it because you want to take down what he said and it works better if you argue he said "defines e new genre".
Anyway I understand what he means and agree with him. Yes it's a shooter but you compare it to almost every other shooter out there and you'll see the only similar aspect between those are, well... the fact that you're shooting. Other than that you have a shooter were you play squids who shoot ink and the objective is to paint the bigger part of the map, you superjump near your teammates at any time, has platforming twists throughout the game, you have a swim-only-in-your-territory ability, you can only walk over your paint, it's colorful, etc. It has enough carisma and caracteristics that differentiates itself from most other shooters.
If you ask to a gamer which is a better racing game, Forza Horizon or Mario Kart 8, most will give you the same answer, "they are completly different, its hard to compare". Most don't even compare a real racing simulator with an arcade simulator, let alone with a colorful kart simulator.
Well the same principle applies in this case. You try and compare, let's say, CoD or Halo or Battlefield with Splatoon, well you'll have a very hard time. Myself, as someone who plays CoD a lot, believe they are two completly differente takes on the shooter genre. So different it would be very hard for me to compare them both. Sure people will have preferences on one or the other, but those preferences will also be very related to the kind of games you prefer or if you are bored at those genres ot not.
Now, for the "defining it" par, well it will depend a lot on exposure, success, how people will like it, even hypothetical sequels. And on these parts it seems to be headed the right way. But, as for the "genre" itself, I'd say it's a different "subgenre" when compared to most shooters on the market.