Brother, part of a healers role is to do damage. It modifies their healing and also helps the group progress. Damage is a big part of healer meta.
OP said no one died, everyone was healed up and the tank had full threat. Would you prefer the healer just stand there doing nothing?
This is also TW we’re talking about. The healer could have been a level 10 resto shaman one shooting every mob with chain lightning.