In Minecraft you can swim up a stream of falling water. The griefers used a lever to open/close the stream down to the ground. I assume one of the griefers had to stay up there and operate that lever.
Since everyone was always online at the same time, it would also be easy-ish to defend the island if anyone tried to build a pillar to reach it. A bow and convenient sniping platforms could knock attackers down. Lava-based murder holes in the underside could be used to defend blind spots.
You could concievably lay siege to it. That would be really interesting to watch. Seems like a minority of the players were in the griefing camp. So the majority could coordinate an attack where they built staging platforms successively higher, with similar attack vectors.
But this would be very hard, since the "griefers" had both lots of resources, weapons and the advantage of the high ground.
