We're discussing proposals to backup FIDO2 authenticator, and Emil from Yubico mentioned that allowing the user to inject their own key material "undermines device attestation, which would likely disqualify those authenticators from high-security applications like financial institutions."

I'm wondering, is that actually the case?

To me, not allowing me to inject my own key material couldn't be an advantage, because I have no guarantee that the vendor didn't keep the copy of it for whatever reason. And if I'm able to set key material, then there is no need to trust the vendor, at least not to the same extent by far.

But I'm wondering whether I'm overlooking something.

The proposal is being discussed here https://gist.github.com/emlun/4c3efd99a727c7037fdb86ffd43c020d#gistcomment-3073739