That page includes the text "Our search results also include anonymized API calls to all major search result providers worldwide".
They source results from lots of places including Google. One way that you can confirm this is to search for something that only appears in a recent Reddit post. Google has done a deal with Reddit that they're the only company allowed to index Reddit since the summer.
edit: I don't think this is a bad thing for Kagi. I'm a very happy subscriber, and it's nice for me that I still get results from Reddit. They're very useful!
Note that due to adversarial interoperability, search engines other than Google can scrape Reddit if they try hard enough. A rotating residential proxy subscription, while pricey, likely still costs orders of magnitude less than what Google paid. The same goes for Stack Overflow. You can also DIY by getting a handful of SIM cards. CGNAT, usually a scourge, works in your favour for this application since Reddit can't tell the difference between you loading 10000 pages and 10000 people on your ISP loading one page each (depending on the ISP)
They source results from lots of places including Google. One way that you can confirm this is to search for something that only appears in a recent Reddit post. Google has done a deal with Reddit that they're the only company allowed to index Reddit since the summer.
DuckDuckGo gets no answers if you specify only results from the last week: https://duckduckgo.com/?q=caleb+williams+site%253Areddit.com...
Kagi is fine if you do the same: https://kagi.com/search?q=caleb+williams+site%3Areddit.com&d...
edit: I don't think this is a bad thing for Kagi. I'm a very happy subscriber, and it's nice for me that I still get results from Reddit. They're very useful!