He's doing it kinda the right way...but no, you will not ruing the heads if you touch them with the tip...at least not in any electronic way. It's more about leaving marks on them...which a bare, metal tip could certainly do.
As long as the machine is OFF...the de-mag won't hurt the heads if you accidentally touch it...and you will most likely, because there is a pretty strong magnetic pull when it is on, so the heads will want to pull the tip. Might not be as noticeable with the smaller de-mag units, but I know my bigger Annis Han-d-Mag really kicks out some magnetism, and it certainly does pull hard.
Also, most of the guides are kinda pointless to de-mag, because they are usually mad of aluminum on most machines of the last 40-50 years. Yeah, the center spindle may be some other metal, but really...I think people over estimate how much magnetism is left behind on the heads or guides from use.
It's pretty insignificant overall...but with the small heads/small format/small track widths...I guess every bit help to keep things sounding the best...just no need to overdo it.
You can actually cause more problem because of the de-mag process if you don't do it right...but even that is not as critical as has been made out to be.
People afraid to move too quick or needing to stop back 3 feet before shutting it off, and all that.
Just move it fairly smoothly, and bring your arm well back before you shut it off. I like using a power strip with a switch...that way with my one hand I can flip the switch on/off while I hold the de-mag with the other. Easier than pulling the plug to shut it off.
I like to make the heads the last spot I use it before I pull it back to shut it off...since that's really the area you want to clean, more so than the aluminum guides.