Well, that and a fastball that could bust a single-pane window.
Look, Andy Sonnanstine, in over 550 minor league innings, put up a 2.76 ERA with a 503-85 strikeout to walk ratio. He throws a fastball/cutter/slider/curveball repertoire at hitters. He also averages less than 87 MPH on his fastball, and has a career ERA of 5.22 in part because he started giving up more homeruns in the majors than he did in the minors. Major league hitters not only make more pitches into mistakes, they also do a lot more with those mistakes.
Milone might be the rare person who can overcome a severe velocity deficiency, but it's really hard to succeed in the majors on command alone.
Left handers have a much easier time overcoming velocity challenges than right handers like Sonnanstine do.
At this point, what's left to prove in the minors? The jump to AAA hitting is not that much of a difference. It's not like he could be that much worse than guys we've ran out there this year, Atilano, Mock, etc. etc.