The pillars are to make it unsailable (like a rocky coast), as you're meant to fly there on a dragon. If a patrol bot kills the cultivator, then you cannot get the achievement unless you kill the cultivator again yourself (like in the arena).
You can certainly fly past island without really exploring them, although eventually you kind of have to explore many of them for the quest progression. You can't really sail past islands though, as sailboats stop at coasts.
I'm glad you enjoyed the game, and I hope you'll enjoy our future updates too!