Reminder: Bending itself isn't learned, the mastery over bending martial arts is. Aang journeyed the world to find masters to train with in order to master the four elements, not learn them, he has always had the inherent ability. Here's Aang creating a massive wave the very first time he waterbends
As the Avatar he has the inherent ability and proficiency to do so, he's the most powerful bender but he lacks mastery & control. That's the whole point of the entire Jeong Jeong arc.
Pavi and Nisha already being able to bend all four elements isn't a retcon, this has always been the case, they're far from masters. The Avatar trains to master them, to learn to control them fully, not to learn the elements themselves. It's always there as long as the Avatar is spiritually/personally connected to the element.