DB or someone operating for the university could have asked him "are you wanting to stay in the college ranks or you taking it to the next level?" and he said "NFL."
simple as that.
the notion JH was "in Michigan's pocket, theirs to lose" is a fucking fan-base created notion that has ZERO reliable basis other than DB saying "he's been interested in the past." JH just as easily may have found out he could make a million more and take it to the NFL than to keep the college route.
let me make this clear: Michigan didn't "screw the pooch." Harbaugh goes to Clemson or UConn or Maryland? ABSOLUTELY. the guy goes to the NFL? he wasn't available.