It's a tricky and delicate subject. Yes, Obama's biracial, but he identifies as black, and that's also relevant to consider here.
50/50 biracial folks in this country also more often than not tend to be viewed as having black skin, in terms of being subject to "routine traffic stops" and other such unpleasant things.
Obama for sure had a unique upbringing with his significant time spent overseas as a youngster, which most black Americans haven't, so that sets him apart in a way.
But having read some of Obama's own writings on race and writings by other black American authors about him, and his historic achievement in the '08 election that many young black folks look up to (however you want to characterize it), Obama's become a touchstone of being black in America, and dismissing his blackness is a tad dismissive of that.