re: SI ranks college football's most talented rosters

He is rating talent potential based on the composite star rating of the assumed roster of each team. It's a forward looking rating. The draft only comes into play when determining if a player is counted or not for the upcoming season (ie, is he still on the team or not).

That doesn't really do much to explain the results, but that's what the link says.

Eta
This is also a Michigan homer, so that probably explains why teams like Indiana, Purdue, and Minn are seemingly way higher than expected. I imagine that subjective and arbitrary boost in rating is largely concentrated in the Big 10.