They do more than that: https://help.kagi.com/kagi/search-details/search-sources.htm...
They're just reselling google searches. With maybe 1% salted in from elsewhere for deniability.
Or at least I thought so?
But with Kagi you can always lower github results if you want to. Or boost other sites that you prefer.
If that is true why hasn't Bing noticed it?
The github bias in google results is huge compared to bing. Just try running the same search on both sites. I find it hard to believe that the Bing engineers simply haven't noticed this.