If you compare Sexton and Jrue, its pretty easy to see how much more technique and IQ/instincts/experience matter than athleticism.I just think the definition of athleticism in basketball peoples minds is skewed towards very specific traits. Reaction time is athleticism. Lateral movement is athleticism. Balance is athleticism.
Sexton probably beats Jrue in most aspects of athleticism, isnt drastically smaller nor does he lack in effort.. but there is a massive difference in their defensive prowess.