This is a excellent point - and pretty much how I considered the question. Against other elite P4P boxers in history I do think it gets quite hard to back any single fighter against everyone. The margins are incredibly slim. I actually thought it referred to fighters operating in their own divisions, in their own time. And only Roy Jones stood out in my lifetime.
Michael Spinks Light Heavyweight RJJ Super Middleweight Thomas Hearns Jr Middleweight Sugar Ray Robinson Welterweight Pernell Whitaker Lightweight