Well, that's the argument. That she dominated her era more than any other tennis player. I'm not even sure that's true for the women, although it's certainly arguable. To me, the most impressive thing about Serena is her longevity, how well she's played at an old age (for tennis). But the reporter wanted him to say she was the greatest tennis player ever, instead of the greatest female tennis player ever. Either she didn't know what she was talking about (that she thought Serena could beat the top men), or else Mac misunderstood what she meant. It wasn't that he went out of his way to insult her, he just responded to the question he was asked.
I'm sure there are people here willing to say that Serena is the greatest tennis player ever. I understand the argument for it, but I don't think I'd be comfortable saying it.
I would put my money on her over Mac though. Mac said he gets winded playing one set. I do think he's better than Riggs was when he played Billie Jean King, but he's just too old, IMO.