It's not wrong for you to treat them as having asked for a compliment. Being explicit about the fact that they asked you for a compliment is bad manners but implicitly acknowledging it isn't. It completely okay that they know, that you know, that they didn't want honest feedback.

I have to admit that this baffles me, but I'll take your word for it.

When it comes to telling whether people are fishing for compliment or seeking honest feedback, it might seem complicated at first but it's not asking for the moon.

Even if we're talking about that particular aspect, it's kind of hard and I'm not exactly being showered with data. I don't actually experience that many people asking for a compliment, do you? Come to think of it, the whole thing may not be as big of an issue. (I know why I have such a strong emotional aversive reaction to it nonetheless.)

I think introverts often think too much of "what's the social custom and how can I follow it?" or go down the extreme of pickup artistry but too seldom go the middle way of finding nonstandard behavior that's completely socially acceptable.

I think I'm basically doing exactly that.