No, not head to head(I hope they don't think that), but they're arguing that he was better overall as a fighter at WW. The competition though is much greater up there due to the sizes so he wouldn't fare as well head to head.
I think he would be more competitve against Duran at welterweight than lightweight. Duran slowed down ever so slightly overall as a fighter, although he probably gained even more ring sense and knowledge around 79/80. Mosley on the otherhand was quicker and stronger after moving up. His peak days as a fighter were his early welterweight days when he stepped up his comp to a certain degree along with his effectiveness. I'm sure Mosley himself would say the same. He had a 5 or 6 fight span where he shined like beacon.
But against the bigger WW's he wouldn't have fared as well as he would the LW's, who he'd almost always be of sufficient strength and size against. I also think Duran looked better against Leonard and Palomino maybe than I've ever seen him look at LW.
I was just talking about Mosley against Duran. Taking away the possible competition at lightweight and welterweight for a moment, Mosley was was stronger at welterweight and also quicker. His peak performances and physial condition and overall effectivess, all better at 147lbs. His prime as a fighter was his early days at that weight. Duran IMO was equally as good for De Jesus III. But while Duran looked brilliant against Palamino and Leonard his power and speed had slightly evaporated. Physically he wasn't quite the fighter he was at lightweight. Duran himself obviously wasn't a full fledged welterweight, hence the reason I give Mosley a better chance at welterweight with the first paragraph above.
I thought Mosley looked poor against Rivera. He's never entirely convinced against good fighters.For every good performance a sub-par one can easily be found. For weaknesses, i would say he just isn't much of a refined technician or thinking fighter. His physical gifts and offensive ability make him a tough fight for a lot of lightweights though.