Minecraft datapack with a void dimension?
Solution 1:
As Fabian already said, commands can not add another dimension.
The datapack featured in the video uses the upper half of the nether. It even uses parts of the nether for terrain generation:
It clones netherrack and nether quartz ore from parts of the nether and replaces those with other blocks (different for each layer, in some layers the air gets filled, too).
Solution 2:
No.
Commands cannot add dimensions.