They might be other planets that they THOUGHT were moons.

Because the One Piece world isn’t super advanced, so they might still think the Earth is the center of everything.

Or there might just be a bunch of moons that got blown up somehow.

but in theory for them to think like that the other planets should be as clear as the moon that "Earth" has. They would have to visualize these "Planets" during the day, for example (just looking at the sky).

I haven’t fully caught up but watever Im sama used to erased that previous Island pretty sure the same happen to those moons

I think some of hese different species came after someone destroyed those moons. Probably Imu.