I would guess the manual states that because of some people who use the gap gauge as a pry bar between the centre and ground electrodes. Definitely not the right way to open up the gap! You've got a good chance of completely bitching the centre electrode of you do that. My gauge (Mac tools) has a "window" that you hook on the ground electrode only and leverage off the ground electrode itself to open the gap.
2011 XLT XTR 4X4 145" EcoBoost Screw Solo Performance Mach-X Eco-Dual Exhaust. Solo high flow down pipe