The description/tooltip is wrong. It increases the parry CHANCE by flat 40% NOT the parry RATE. I also tested how it stacks with [Survival Instinct] and it added exactly 40% to the parry CHANCE (not RATE).
As you can see on the screenshot, the parry rate is 0 but the chance to parry is 40%.
There is nothing wrong about this description. Parry Rate is increased by 40%. The value you are talking about is Parry. However, to remove confusion, we will change name Parry Rate in character frame to Parry.