Was it a secret area?
If it was a mod, it was far more complex than any so far.
This was all quite confusing.

But it would’ve been a huge effort to recreate all this outside the game.
So what was it?
We hope everyone had a good time, we certainly did."

But the end result is great.
They have another mod coming too.
Unless this reveal is itself a prank, in which case: got ‘em.
The creator of the DN3D level evenreleased it publicly, though that download link is dead now.