I mean, the description literally doesn't state any specific damage source. It just says the wearer deals +2 radiant damage.
You might subconsiously add in the qualifier of "With attacks" but no, the wearer just does +2 radiant damage. So all sources of damage they deal... Have +2 radiant damage.
Including Warding Bond (Which due to the way it works, it counts as the damage being reflected to the caster as coming from the recipient)
