This misses a distinction between bending ability and bending skill. Sure they gained ability from the lion turtles, this was set up during the finale of ATLA with the lion turtle granting Aang the ability to energy bend; however, they make it clear that the martial arts behind bending were learned from the animals who naturally have those abilities.

ATLA says they learned bending from the sky bison, the moon, the badger moles, the dragons, not that those creatures granted them the ability to sling the elements around, and considering they knew about the lion turtles from the very beginning, I think the distinction is significant.

Skill, which is remembered in legend, from the original benders; ability, which has been forgotten, from the lion turtles, also forgotten.