Guardian damage doesn't seem to do anything.
Guardian Damage (I think I named it "pet damage" or some shit) is poorly explained on my part. It works as a bonus multiplier to a minion's damage. It does not directly set their damage.
Specifically, each time a new room is loaded the minion's damage is calculated as
totalDamage = (baseDamage * bonusMultiplier) + baseDamage
, so even if you set the Guardian Damage setting to 0 the minions will still deal their base amount of damage. Their damage is also only calculated when you enter a room (or when a new minion spawns), so changing it mid-room won't have any effect until you leave the room
Would you rather be able to directly modify their final damage amount instead of modifying the amount of bonus damage? Now that I think about it that'd probably be more clear. I can change it to that next time the game updates