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.
