They have a point. . .if you only carry one knife.
I carry two, one in each front pocket. I carry them regardless of whether I'm packing.
But, I have a totally different response to gun grabs. I don't automatically go for my knife or other weapon. I respond with low- and high-line strikes and redirection before I go to a secondary weapon. I'd only go for a secondary in a really bad situation, since that takes up valuable time.
Plus, in such a close-quarters engagement (you're already grappling), a knife is just a hindrance, IMNSHO. Unless you actually plan on waiting for your attacker to bleed out, it's going to be difficult to get to structurally-debilitating targets (flexor tendons, bicep/tricep, pecs, traps, sartorius, quads, etc.) AND you're going to be reducing the ability of what would be an otherwise free hand to grasp/grab.
A neck knife is notoriously difficult to draw, unless it's outside the clothing. You got some guy (presumably) trying to go for your gun and you're going to, what. . .snake your hand up your shirt, grasp and deploy your knife while you're getting hit, kicked, punched, jerked around, thrown to the ground and bitten?
Sorry, not trying to be a wet blanket, but I see many problems with the very basis of the theory.
The Professor