It does seem that currently Thickskinned and Thinskinned are doing the exact opposite it what they should be doing.
1. Here we see that n4 is defined as your blade/blunt guard + 15. In other words, your defense.
int n4 = 15 + this.getParentChar().getMeleeCombatMod();
2. If you have Thickskinned, your defense gets smaller. If you have Thinskinned, your defense gets larger.
if (this.ParentChar.Traits.ThickSkinned.isSet()) {
n4 = (int)((double)n4 / 1.3);
}
if (this.ParentChar.Traits.ThinSkinned.isSet()) {
n4 = (int)((double)n4 * 1.3);
}
I don't think any further examination is needed to conclude things can't possibly be working as intended.