They're still ultimately scraping other engines pages to get the results though. The last time this was discussed an engineer on their team told me that they were building their own index internally, but I couldn't get any additional information about that and it doesn't appear to be in their docs.
https://help.kagi.com/kagi/search-details/search-sources.htm...
Why is it bad if they're using other engines to produce results?
I like paying to invest in a new sustainable ecosystem. Using SERP API doesn't feel like that to me. See Brave for a counterexample.
If engine A uses the results of engine B, and engine B uses the results of engine A, there is a problem :-).