Interesting. I have not had any issues using their engine even with the issue with Bing's API, but you are correct that they use Bing's index. Given that there are only four indexes to choose from, that isn't too surprising.
I actually switched to them when I saw that DuckDuckGo was about to start providing 'AI assisted results'. I wanted to ensure I was using an engine that actually respected my privacy and didn't harvest my data for slop.
Anecdotally, I can confirm that the results I get from SwissCows are very different and usually better than the ones I got from DDG. So I wonder how much of Bing's API they use.