As someone who uses Kagi, I think it is absolutely not accurate to say they are a search engine rather than a meta search engine. Most of their results come from the other engines you listed.
As per the first sentence on that page, they do. It's just not their only source, and from my experience it is far from the main source. You can see "% of unique Kagi results" on each search; these are the results from their own index.