Okay, no need to exaggerate people o_O
Yes, you DO take damage while knifing someone using commando... but they take LESS damage... Or rather, less damage is registered because of
*Lunge Lag*
I shot a guy twice with the m1014 (w/ stopping power) while lunging, got hitmarkers twice, yet it didn't kill him... why? because on the killcam, it showed just one shot, and half of it went wide, allowing him to survive. This is Lunge Lag.

If a person knifes you while using commando, his "extreme" speed causes a sort of lag where he is 3-4 feet farther up than you see him to be on the screen (also known as ping, or latency).