Do you have a citation?
Just from my quick research this seems to be a research partnership, not a user data sharing partnership.
“In this context, Qwant remains in control of its technology, including its algorithm, index and client infrastructure, without collecting personal data, to better secure the privacy of its users and the confidentiality of their research.”
Being open source offers zero guarantees that meddling isn’t taking place. It’s configuration, local adjustments and what it’s partnered with are completely unknown and they can all introduce bias.
For example someone could simply block all traffic from a particular GeoIP range and even say “it’s for security reasons” but the effect will be the open source crawlers don’t even see the information to crawl it.
This last argument I don’t fully agree with but I think it has some merit:
Open source interpretation of results are also vastly easier to game. Now that’s a dangerous argument to make because it’s similar to saying security software should be closed source to make it harder to break but I think the difference is security “just” has to protect a narrow range of use cases so it’s perhaps doable to lock it all down where as search rank interpretation is a boiling mass of constant evolution. (at least a lot more so i’m imagining)