That's not totally true. FX-322 first multiplies the supporting cells and then transdifferentiates them into hair cells. Audion's drug does the transdifferentiation without multiplying the supporting cell first.
They both cause the supporting cell to transform into a hair cell. FX-322 just does it with depleting the supporting cells. The main mechanism is the restoration of the hair cells.
FX-322 does not transdifferentiate new supporting cells into hair cells. It causes supporting cells to divide. Some of the new cells are hair cells, some of them are supporting cells. You're suggesting the new hair cells were supporting cells before they became hair cells. That's not the case.
Source:
http://www.cochlea.eu/en/research-lines/regeneration