I found no headline or article supporting that Deniro has Bipolar Disorder. Does he?
It looks like the headline got changed somewhere along the way. As I read the article, De Niro understands what David O. Wallace goes through with his bipolar son, either because he saw them interact, or because he goes through something similar with his own kids. By that interpretation, De Niro has a child who's either bipolar or troubled in some other way.
Of course he could be bipolar himself, but if he plays the parent of a bipolar child in the movie and is talking about how real-life experience made it easier or richer or satisfying for him to play the role, he wouldn't be talking about having the condition himself.
Sure looks like he is claiming he is bipolar...better than being bisexual or bicoastal