When you use as preset for your superflat world for example end_portal blocks, the game states that it uses air, however it actually uses end_portal blocks.

How to reproduce

1. Use the following superflat preset

   3;minecraft:end_portal;1;
   

   The game should now display only one layer of air

2. Create the world

You should be in the end. This means it actually placed end_portal blocks.