Technically, that's argumentum ad absurdum, wherein he ignored other important details to present an absurd conclusion as a grounds to debase the argument (when the argument itself precludes such a conclusion from its given facts).
Tanks are tanks because they have a greater ability to mitigate incoming damage, and are built to generate more enmity. Those are the two fundamental goals of tanking (because if you do those two and those two only, barring any failure to perform on the other party members, the group will succeed). In the current paradigm, the thought seems to be that Parry isn't as valuable because you'll gain more "proxy" mitigation by killing the mob(s) faster, preventing their damage. If that was the only source of mitigation? The highest DPS should be the tank.Hell, outside of tankbusters, there really isn't much benefit to bringing tanks to Alexander Savage.
I'm not denying that damage contributes to mitigation. What I'm trying to understand is whether Parry's scaling is really so poor that the mitigation by damage increase from not having Parry is greater than the mitigation that having Parry would provide.
RNG arguments don't really fly, as long as the presented rate translates (roughly) to "real world" performance.
EDIT: I also am not denying that current encounter design might be counterproductive to the effectiveness of Parry, as there is a very clear lack of outgoing damage to make tanks value mitigation. This is more just asking about general job design for tanks, and whether Parry really is such a subpar stat.