That is only if “the healer casts” is defined as “the heal comes from the source: the healer” or “the healer hard casts the healing spell”.
If it was hard casting it would also screw over resto druids for example, since a HoT is not a hard casted healing spell either, but a buff that heals over time, too. Same with resto shaman totems.
Basically every healer would be screwed if the set bonus was only “hard casted heals”.