Exactly, 'smartness' is meaningless. You can accomplish x task with y practice, applies to any task.
Meanwhile, intelligence research continues to be one of the more fruitful (and reliable) fields of the social sciences
This might also be an either-or situation - i.e. some people specialize in the ability to quickly grasp a wide variety of knowledge at a relatively shallow level, others specialize in in-depth knowledge of a particular subject, and it's impossible to be really good at both.
Maybe to a limited extent, but most of the difference is likely natural ability.
Wisdom is knowing not to put it in a fruit salad.