Appa airbends in other ways too. That thing he does with his tail is more than just him swinging his tail. He just starts the wind like an airbender and then controls it with his chi. This is the main reason i believe they learned from flying bisons. Finally, how many badgermoles do you guys think there are. I wouldnt be surprised if there are more since their were a ton of flying bison in one of aang's flashbacks.