And how? Because they were both popular games on the internet at some point? lol
I appreciate that Splatoon was an honest attempt at Nintendo making a kid-friendly shooter, and it certainly did so well. But frankly, out of every awesome game on that list, does Splatoon really bring enough to the table to stand its ground? I certainly don't think so. It's a good game, but GOTY I really don't think so. Maybe it was Nintendo's GOTY but certainly it isn't objectively better than some of the amazing stuff on that list.
Undertale affected me and got me hooked in a way that I haven't been hooked in a really long time, whereas I had trouble getting into some of these other games. So for me, it's GOTY. I don't see how Splatoon really did anything revolutionary or emotionally affected people at all. In fact, it's hard to play for more than 30 minutes at a time given there are only 2 maps to play online at any particular time.