Looking at Grapple A 67-70, Krush B 51-55, Puncture B 51-55, Slash B 51-55, Slash B 56-60, Tiny D61-65, Tiny B 71-75, and especially Sweeps C 67-80,
Must Parry for 6 rounds? Stumbling off balance or out of position for 60 seconds?
That's something you usually don't see outside of cartoons or slapstick comedy, someone doing the "woah, woah" dance for a minute.
I strongly suspect that when that crit was written, either must parry was stun, or the author thought it was. . .which is substantiated by the places in RM2 where it's stated to be stun. Spending 6 rounds in a mildly stunned state makes sense, not quite fully at stunned but at least a bit loopy.
That said, I think that the general trend in writing was towards it being not stun, and certainly by the time RMSS came out it was flatly and officially not stun. . .so I'd say it's not stun, and hasn't been for quite some time. . .
I still find that result an amusing image to think of. . ."Woah. . .woah. . .woah. woah. . " can't wait for it to come up again in play.