I can't say anything about the specifics of this treatment, but in terms of their ability to fully benefit from hearing, it would depend on when they became deaf, and the severity of their deafness.

If they were born deaf, or lost hearing as a young child during the language development stage, then it would probably be a long adjustment. Things would just be noise and it would take a lot of training to distinguish sounds, speech, etc. And unlike a cochlear implant, you couldn't just take it off to give your brain a rest.

If they had hearing loss later in life, or some residual hearing, then they probably have a better chance of re-adjusting to hearing.