Hmm... from https://kagi.com/faq#Where-are-your-results-coming-from

> Our searching includes anonymized requests to traditional search indexes like Google and Bing as well as vertical sources like Wikipedia and DeepL or other APIs. We also have our own non-commercial index (Teclis), news index (TinyGem), and an AI for instant answers.

Their founder on Discord said that they trialled turning the Bing API off at one stage, but apparently users started complaining that their search results went shitty - so I imagine it's used more heavily than that post would imply.