It's actually a ArmorProject IP, owned by Horii, Toriyama and whoever manages Sugiyama's stuff now.
And even if it were a 100% Square Enix IP (Armor Project just licenses it to Square most of the time, there are DQ games which were published by Nintendo, for example. Like DQ11S or DQ8 and DQ7 on 3DS). Xenoblade was developed inhouse, Nintendo didn't buy the IP. Big difference.