Well, that’s a lie. As Orb of harmony heals 30 hps and Caduceus heals 55 hps.
You know what isn’t a lie? Zenyatta’s dps is WELL over twice mercy’s dps and he can keep doing it while healing. And that’s before factoring in team mate dps from his orb of discord.
That said, Zenyatta was supposed to be kept in check by his low mobility and survivability, but when he’s teamed with either Brig or Baptiste, they can keep him alive all day.