According to the video, you need vacuum because in air the surfaces will discharge before they are separated to the point where the potential large enough to generate x-rays.

I tried the glue on envelopes and I tried sugar cubes and have seen both of them emit a faint blue glow. And I assure you I was not standing in a vacuum chamber when I tried it. Just try it yourself in a dark room. It actually doesn't have to be exceptionally dark, it's pretty visible.

And there are of course also videos showing the effect, here with duck tape outside a vacuum chamber. [1]

[1] https://youtube.com/watch?v=54R6q2_-4Yo

I think they specifically meant x-rays, rather than blue light; the x-rays would require more energy, so their explanation makes sense to me.

You are right, if you are after x-rays specifically this might not work. But I can't see x-rays so I am happy with some blue light.

yes, you are generating ~430nm light

x-rays (0.1-10nm) are much higher energy and need a greater potential to be created, which can only be done in a vacuum

*duct tape (plumbing)