As to not get too split-hairy here, I just want to focus on the definition of a symptom vs the phenomenon of masking.

You (and presumably the other commenter) are talking about masking. Were happiness to be a symptom of depression, like, say, a fever is a symptom of the flu, you'd have to evaluate every person who displays happiness to test for depression. We know that's not remotely true.

That is because, as a symptom, people would put on a happy face because they're depressed, and not to hide their depression. That's the difference.