They are different conditions (albeit ones that have shaky scientific bases as 'diseases' due to the subjective nature of diagnosis).
Prozac/fluoxetine is not a prescription drug used for bipolar illness as the excess serotonin can in fact trigger manic episodes and it is thus very dangerous in bipolar patients
He's not saying that mental illness isn't valid, or that the people with depression aren't suffering. He's suggesting that the increase could partly be caused by the immense advertising campaigns surrounding anti-depressants; this is in no way de-legitimizing depression, just suggesting a possible cause.
Other industrialised countries - Australia, the UK, France and Germany, etc have also experienced a rise in patients under treatment for these conditions, in lockstep with the US experience, despite this difference you suggest is responsible
The author is trying to claim that many (most/majority?) cases of depression are made up and hyped. They are trying to claim that it's impossible for there to be that many actual depressed people now.
But turn it around. How many (openly) gay people/couples are there today? Millions! But there weren't that many in 1955! So what happened? Was it all media hype? <sarcasm>Or are we expected to believe that all those gay people were suffering in silence in the 1950s?!</sarcasm>.
Of course all the gay people were suffering in silence in the 50s. People don't doubt that. So why the doubt about mental illness?