Now we are in the translation forum, the Japanese version says:
Reduces your damage by 20%. In addition, the amount of increase in maximum HP due to the increase in VIT and the amount of increase in damage dealt due to the increase in STR are corrected unique to the Tank roll.
While the English version says:
Reduces damage taken by 20%. Furthermore, grants a bonus to maximum HP based on your vitality attribute, and a bonus to damage dealt based on your strength attribute.
Underlined is where there seems to be a significant difference in what they are saying. Emboldened it's saying there is a bonus while the Japanese version is not saying either way whether there is a bonus or a reduction, but it's obviously implying a reduction because the tank role is supposed to do less damage than the DPS role.

Perhaps this is intended but I doubt it, because people who do the maths can see there is a reduction to a tank's strength attribute compared not just to DPS, but even compared to healers.