I said the same thing when this retheme was first announced (got multiple old posts in the other thread about this very topic about the animal designs not matching). However, it has become evident from some of the art that they're going to be incorporating animals with more human-like proportions. You can see it in the backgrounds of the two pieces of art above, there are animals that have proportions similar to the characters in Splash.Well, it's more that most of the animals in Splash Mountain don't look very much visually like the ones in The Princess and the Frog. I could see the Imagineers thinking that the anthropomorphic geese would clash with the non-anthropomorphic spoonbills.
I guess they've gone with the idea that there are other animals like Louis out there who have trained themselves to stand upright and behave/move like humans.
To be fair, even a few of the animals in Splash varied in how they were proportioned. Some of the frogs were proportioned like humans (like Brer Frog in the first lift hill sitting in his chair), while others were more "normal" looking frogs squatted on their platforms on all fours. Same goes for some of the alligators and turtles.