In practice, "AI" is used as an umbrella term that definitely includes that kind of stuff. I can't speak for YC, but IMO Semantic Web work should probably fall under the AI umbrella in this case.

(Edit: in the interest of full-disclosure, my startup is doing a lot of work with Semantic Web technologies, particularly in terms of applying them to Enterprise knowledge management)

It's not information retrieval if there's no information to be retrieved in a vertical market. With the right incentive structures and business model, new vertical data sets can be generated, which enables new applications of existing algorithms.

Hopefully the term AI still includes cybernetics (e.g. Biological Computer Lab), and is not limited to currently fashionable neural nets. In cybernetics, the vertical observer is as important as the observed "data".

I have a natural language question answering system build on top of information repositories.

Understanding the intent of questions is an AI topic for sure.