It seems unlikely to me that anyone is trying to play you for a fool by claiming sobriety when they are really on anti-depressants. Most people do not consider doctor prescrbed anti-depressants in the same category as the drugs or alcohol they used and abused recreationally. You can look at it that way if you want to, but be aware that most people don't and they don't consider themselves to still be 'using' when they take their doctor prescribed medications.
No one is lying to you. You have a different standard.