I think there's a lot of truth in that. Yes, you can have a genuine chemical/biological imbalance, and it may even be heritable... and it's totally appropriate to apply "illness" to that. Most other stuff? More of a fabrication, I think. For example, in the time of C.G.Jung, it was considered a natural thing that some people had a "Melancholic temperament." By today's standard, that would be labeled "clinical depression" and treated.