Forgive me for being reductive, but aren't these leaked accounts a lost cause? The vulnerability in question is attackers being able to log into user accounts with leaked credentials. The only mitigation for this is to lock out users identified in other password breeches and reconfirm identity out-of-band, like through a local bank branch, add a second factor like a hardware token, or use restrictive heuristics like IP geolocation consistency between visits.

If 3 attempts per hour is enough to gain access, then it doesn't seem attestation can save you. I imagine a physical phone farm will still be economically viable in such case.