From the way the manga is written, Devil Fruits seemed to be extremely rare back in that era. However I find it hard to believe he'd be superior to Whitebeard without one.
That's more likely than almost any other scenario though. With all the comparisons between the two, Shanks being in the east blue, having that particular fruit etc..
Still makes no fucking sense because even Whitebeard or any of the other people who knew Roger personally never comment on Luffy's fruit being used by anybody else. Not even Shanks who found that fruit in the first place.