That said, this still feels odd to me. How common is it for a player to leave early for a pro career outside of the NBA?
It's quite common.
There are only 60 NBA draft picks every year. A good number of those go to seniors and foreign players. Only the 30 first round picks get guaranteed contracts and it's not uncommon for 2nd round picks to never see an NBA court.
Still, 137 underclassmen left school early last year. Given those numbers, many of them had to know that they were unlikely to be going to the NBA.