Why do they talk about isolating it from the air, but not from conductive material around it?

Surely air is the easier of the two to do. If you're concerned that it's working by pushing on the air around it, just put the thing in a box. Measure the net force on the box; if it's pushing on the air then that force will be 0.

My automatic assumption from the way they describe the machine is that if it's not breaking the laws of physics, it's probably pushing on conductive material nearby with an electrostatic force. You can mitigate that with a similar approach by surrounding it with a faraday cage I guess, but that part at least seems somewhat more tricky.

Disclaimer I know absolutely nothing about the issues involved but that's my immediate take on it and it seems weird to me that they don't mention anything about isolating it from conductive material.