Ground is a very relative concept in electronics. You are correct about earth ground, but if the case is unplugged and you touch it, you are still grounding yourself to chassis ground. Chassis ground is the important one for this since the danger isn’t electrocution (assuming the PC is unplugged) but electrostatic discharge (ESD) that can damage components.

This is still a massive simplification of the concept of ground.

And if there’s ever a brand integration, he’ll spout their marketing drivel uncritically and try to pass it off as science. I can’t trust him anymore.