Here's what he said:
Oh OK, so it affects the behavior of people around it,” he said. “That’s interesting, yeah.”J.K. Rowling doesn’t mention if Harry being a Horcrux contributed to the Dursleys’ hatred, but has written about the Dursley’s dislike for Harry on pottermore.com.
Radcliffe continued, “Yeah, that’s definitely the first time I’ve heard that one. It’s definitely interesting because it sort of posits a world where these people only known to be a certain way. Actually if you remove him from the situation, [they] would have been quite a nice, friendly family.”
Having been there for Richard Griffiths’ (Vernon) and Fiona Shaw’s (Petunia) performances,” Radcliffe said, “I feel like it was more they just hated having this kid around.”
“But I think that’s definitely a cool theory,” he added. “Has anyone asked Jo Rowling what she thinks of that one yet? I feel like that one ― that’s definitely a good one.”