They do have their own crawler and I believe supplement from bing only when necessary. Something like 98% of results are from their own index across all users. They actually have a breakdown for every search.
I think they actually have the largest independent index outside of Google/Bing right now.
Others that exclusively use their own index are:
Mojeek (results aren’t great)
Kagi (result are pretty good imo)
I use Kagi too, it’s surprisingly snappy! Like seriously impressive for a small org. They talk about speed optimisation being critical for them as well. I find the result to be excellent as well. A true Google replacement/feels like Google in its prime.
I believe they have their own index and bot as well?