I don' think they're saying it's the next Mario Kart - they're saying that what they want KS:Rivals to be to the X1 is what Mario Kart is to the Wii/any Nintendo hardware - an instantly recognisable exclusive that doesn't really age and is still going to be played years down the line.
Example: "You've got a Wii? Got Mario Kart with it? No? Go out and get it, you dumb ******! It's ****** fun!"
Now replace Wii with Xbox One, and Mario Kart with KS:Rivals. THAT's what they want. The sort of it's-mainly-for-casuals-but-I-like-it guilty pleasure that gamers will have a quick go on years later when they just want a quick blast of something rather than getting bogged down in some intense FPS or other.
Still a lofty goal, but potentially achievable, if you set your mind to developing it like that from the off, rather than starting off trying to be all hardcore, then realising you've not got that sort of product.
Edit: I'm still disappointed that Rare couldn't get this ready for X1 release day
but better to delay it rather than release a substandard version.