BookBarbarian
Expert Long Rester
Pretty sure that is a misimpression. It says "When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together."
Compare to the barbarian's Brutal Crit, which says "you can roll one additional weapon damage die when determining the extra damage for a critical hit with a melee attack."
This is correct. It's not crits in General that are limited to one die, just special things like Brutal Critcal, and I think the Half-Orc ability that operates similarly to it.