I think I know what he was trying to say and there is truth to his statement. He wasn't talking about losing more hearing either. That, of course, would be nonsense and it is.
What he meant was that hearing is like a muscle in which it must be used to get the maximum benefit from it. In other words, you need to continue to use a HA, CI, whatever device to keep what hearing one has left "sharp". As an extension to this thought, to hear and understand speech and by same token speech itself all require the constant exercise thereof to be good at it. So, if one doesn't use any kind of device to "hear" and exercise it, then one will become more "deaf". All senses must be used to be useful. One can't just turn it off for a while and expect it to be good as before when turning it back on.
A good analogy is somebody decides not to walk for a month and just use a wheelchair instead. At the end of the month, the person tries to get up and walk. Lo and behold, they find they are very weak and cannot walk very well. They will have to build up their strength and muscles before they can walk like they could before.