"Don't make me feel worse," said Harry. He told Hagrid about the pale boy in Madam Malkin's.
"—— and he said people from Muggle families shouldn't even be allowed in ——"
"Yer not from a Muggle family. If he'd know who yeh were -- he's grown up knowing' yer name if his parents are wizardin' folk. You saw what everyone in the Leaky Cauldron was like when they saw yeh. Anyway, what does he know about it, some o' the best I ever saw were the only ones with magic in 'em in a long line o' Muggles -- look at yer mum! Look what she had fer a sister!"
I'm not sure what "what does he know about it" in this sentence is trying to convey. It's not a question, but with a structure of questioning. How should we understand it in this case? How should we relate it to the later context: "some o' the best I ever saw were the only ones with magic in 'em in a long line o' Muggles..."?