Oda is talking about whole series in entirety and not on the basis of arcs.
The strawhats are the protagonists.
A series can have multiple protagonists but also multiple deuteragonists.
On a scale of One Piece, a deuteragonist can be found in every arc usually the one who's the centric character of the arc flashback.
Overall across the story, Zoro will always be the second protagonist, that's just the overarching build up for Zoro.
Doesn't make him the second stagetaker every arc tho.
Some people can't comprehense the fact that one piece is a stage-flexible story, it is only natural to have multiple protags, multiple deuters and multiple antas in such a story.