Rocks should have been above him though, which makes me think that statement should be about his general power + ability as the translation suggests. Regardless of if he was the strongest at one point early on, it seemed clear that he wasn't by the time of God Valley. Roger, Garp and maybe Roger's crew beat Rocks, WB, Big Mom, Kaido, Shiki, and more. If Whitebeard was the strongest one around there's no way to make sense of this.
WB may have been stronger than Roger in the past, but Roger definitely caught up by God Valley at least. I think they were equals from that point on.