The Sandy-Grey Void erected by Maroppa wasn't the stable kind created at the Sanrey Plains. No, it was the kind capable of swallowing the whole of Sumatra when left as is. And obviously, having been through the era of Mudropper invasion, Sumatra Continent had a few internal defences prepared against it.
The external defence was obviously the Continent's border, a shallow region that was immune to the effects of the Sandy-Grey Void. All Continents had this border, which was a layer surrounding them and actively protected them against the Sandy-Grey Void.
The internal defence, especially on Sumatra Continent consisted of two: Empyrean Tentacle and Parute Tree.