It's relatively common to end an affirmative sentence with a question mark when your intent is to say something you consider to be clearly right, as opposed to the thing you're replying to, which you consider to be clearly wrong.
I happened to search what it could mean before you answered and got many conflicting answers like yours and [0]. I guess we can only know what the author meant here if he clarifies it himself.
What I said is a statement, not a question. The question mark is not a mistake. What else could it indicate if not a question? Perhaps it was marking my general sense of confusion in response to the original comment. Or it could also be adding a hint of sarcasm. What would you like me say? Do you not approve of my use of it?
Throwamon's explanation is good too. When someone asks a question with an obvious affirmative answer, a popular answer is an inquisitive "yes?". Only the thing being questioned in that case isn't the answer, its the doubt in the mind of the speaker who asked.
I suppose it may be possible to become dependent or relent on these substances, because not being depressed is better than being depressed. Assuming minimal other side effects... Probably best to skip the moral panic.
