I am a customer and I learned about Kagi here. I assume many people are on the same boat, so I wouldn't be so sure about that.
Perhaps I should have said "target customers" where I said "customers", I don't know. But it should not be surprising that "being an HN user" correlates strongly with "finds out about things on HN".
I would strongly bet their primary source of customers came via a HN referral.
We are literally their Target Customers.