It is also skewed towards what slightly-beyond-beginners use; many true beginners haven't discovered it, many experienced people more easily find the same answers without posting.

I'm not convinced that SE activity measures difficulty. But it certainly is affected by much more than just popularity. I could imagine it measuring the total API surface, as you can ask more questions about larger subjects.

It's certainly a deeply flawed measure, but then pure search engine hits like TIOBE are likely much worse.

To be clear, I don't think it measures difficulty, but difficulty is necessarily a driver of the metric. A complex language is going to generate more questions than a small, intuitive language.

Specifically, complexity will generate more questions than difficulty.

No, it can’t by definition. Complexity is a type of difficulty. :)

It's very good of measuring documentation quality. As in, the better your documentation, the less activity there will be.