When you create a world with the floating islands world generation type, there is ALWAYS a single island that is far away from others. This resembles the end dimension, however before 20w21a, the floating islands option didn’t do that and there’s no sign of them changing it on purpose.
MC-184747
The “floating islands” world generation type generates a single island far away from others
Resolved
Cannot Reproduce
4
Bug bug
(Unassigned)
Confirmed
Low
Custom Worlds, World generation
20w21a
Retrieved
This is because it uses the End world generation. You can even see there is end stone under neath the island.
Also note that this feature is experimental as noted in the snapshot information "In this snapshot, we're adding support for custom world settings and custom dimensions. This is an early release to hear your feedback about this feature - do note that this support is considered experimental and unsupported."