Fotification and immunity to crit


So, I'm having a discussion with one of the players in my group, who says, that the heavy fortification armor enchantment, confers immunity it critical hits, just like that of an undead or construct. I tried to tell him, that by having this enchantment; it did not remove your vital organs or slowed your respiratory system down. The player says that is does indeed make him immune. While I try to explain that negation of a critical hit is not the same as immunity.

DMG p. 219
Fortification: This suit of armor or shield produces a magical force that protects vital areas of the wearer more effectively. When a critical hit or sneak attack is scored on the wearer, there is a chance that the critical hit or sneak attack is negated and damage is instead rolled normally.

The ability in mention, which my player wants to be immune to, is the third ability of the PrC Wordbearer from the 3rd party book Quintessential Wizard. Which states that

"Creatures immune to critical hits are not affected..."

But at the same time the generel description says:

"The energies and effects caused by a word of power draws their power from ancien, primordial sources that predate the creation of the univers. Thus, they scythe through magical barriers...

So I'm asking this great community; does heavy fortification confer "immunity" to critical hits or does it simply negate critical hits from melee or ranged attacks?

I have had a hard time clearly understanding whether or not, Heavy fortification would block, Word of Power, Pain from the wordbearer PrC from Quintessential Wizard.