Slughorn's surprise at Muggleborn talent may lack any intrinsic bias whatsoever, if interpreted a certain way. It would be easy to acknowledge a legitimate disadvantage in being exposed to magic at a much later age. People are surprised when minorities succeed in America not necessarily because they are racist, but because they are educated enough to acknowledge that they have been automatically disadvantaged by a system that has been in place for a long time, and that it would be harder for them to succeed than one who did not face those difficulties. Slughorn's estimation might be no different.