They don't "know that he could hit MLB pitching".
They "know" that he has the ability to make the proper adjustments to hit MLB pitching. Kind of like JBJ, who struggled initially, but also shows the perpetual ability to adjust to what pitchers are featuring, and become productive.
I'm not sure how people don't understand this concept. The point is, he made the biggest adjustment in the minor leagues (A+ to AA), which indicates that he is ready to make the adjustment from AA to MLB. AAA is kind of a tweener in the development phase. Once you reach AAA, you're either organizational depth or you're ready to help the MLB team. Very little development occurs in AAA, and by the time a player reaches AAA, you generally know if they are organizational depth or an MLB option.
I don't understand how this is so confusing. Once you have made enough of an adjustment to post a .900+ OPS against AA pitching at the ripe age of 20, you've got the tools to make the same adjustments at the MLB level and help the big club, regardless of time spent in AAA.
AAA is not a development stage. It's a stage where you stay sharp.