TIOBE is meaningless - it is just search engine result numbers, which for many search engines are likely a wildly inaccurate estimate of how many results match in their index. Many of those matches will not be about the relevant language, and the numbers probably have very little correlation to who uses it (especially for languages that are single letter, include punctuation in the name, or are a common English word).
No point asking them to justify why they have to ask, they probably don’t even know. Just say “Sorry, I don’t give that out”. I’ve never had a store push back after that - they probably get it all the time.