If say you attack a unit next to a HB, you deal 50 splash dmg to the HB, the HB takes the same amount of dmg with the guardian shield up. I was able to easely kill a honey badger with his shield up by attacking a building next to him. Works the same vs the ultimate HB as well.

i only tested raiders splash attack, not sure if its a general rule i would assume splash in general isn't included in the parameters but both the first raider and the upgraded raider splash bypasses guardian shield dmg reduction