Actually the Sky Bisons have been able to just straight up fly from day one. This isn’t a new ability, just something we’ve not seen a human bender do before.
Also: Remember how Aang never learned the final technique that would normally allow him his master tattoos because he’d got them early for inventing a new one?
We know that’s not flight because the ancient monk’s technique was considered a legend… So we still don’t know what that actually is.
I’m betting it’s the spiritual projection thing that Jinora keeps doing
Didn’t they say Aang invented the air ball ride thingy? Maybe it’s that.
Remember how avatar Wan could fly with air bending alone? Like, it was represented by a cloud and looked like a flying nimbus. Maybe it’s that.