Didn’t the SHs travel all around Alabasta in like a week or less on camelback?
In some SBS oda confirmed that the OP planet is called the blue planet, and it seems to clearly just be the earth(there´s no saint ¨earth¨ for a reason, as is Naruto´s planet.
The difference is that countries like Alabasta appear relatively small in comparison to the Grand Line, let alone the seas, let alone Red Line.
The OP world has like MILLIONS of islands, some are small, some are mid sized but some are legit countries - WITHIN the Grand Line.
Oda can be criticized for a lot of things but world building isn't one of them - that includes all the islands.