I boated out to the area about a week ago, and nobody has claimed it yet that I saw. When I posted this, I was pretty new on the server still. Since then, I've come to the realization that there are a lot of ocean locations that could be suitable for a rapture build; this was just the first one I found, and it was close to my base.
If / when the server updates to 1.13, I was thinking this might be an idea to revisit myself... maybe.
If you want to check it out for yourself and decide, just hop into the NTN and go as far east as you can (past Tetley Junction to the end of the line), then go through the door (labeled with my name) and take that tunnel to my nether hub. The center portal is my base. Back in the overworld, exit the cave the portal is in, turn around and head East. My base is on the coast of this ocean. Boat out for a while and you'll see the 2 islands in the screenshots.