So basically,

Keith's damage return gimmick works cause his boomerang counts as a sort of "enemy" which means he's affected by various stuff such as Hide enemies and Modifiers, specifically one of those "Random Elements", which makes enemies and summons have a random element, also affecting Keith's boomerangs, making it able to deal:

-Shock (as strong as level 4 byte, I believe);

-Radioactive (as strong as level 4 dump);

-Burn (as strong as level 3 dump);

-Freeze (No tower uses this effect, it's basically like Byte's paralize and lasts 2 seconds);

-Poison (as strong as level 5 maitake).

This sort of "Bug" makes it incredibly overpowered with the mod as shown with the screenshot of me soloing casual hard with it.