I'm pretty sure he knew about that because I told him, but honestly on closer inspection I may have confused myself there! You can increase corruption by increasing damage output, but what I told him was the "original damage" that corruption is based on is the post-buff damage value before any modification by defensive modifiers like shields.
So where the manual says "damage done" it doesn't mean the final amount inflicted, but the total modified damage output of the weapon. I'll do something to clear that up. (I was actually planning on adding a section to the manual that goes over the order of attack operations, because what used to be pretty simple has gotten quite complex over the years with all these new mechanics...).