> By design, you cannot create a backup of passkeys on a hardware key
In fact, I think it's technically possible. But it's true, as of today I don't think anybody supports it.
I was experimenting on solokeys with ios, in principle we could backup Passwords (the ios/mac app) into a solo key, space permitting, including passkeys, regular passwords and totp (not wifi passwords). I believe the same is for Android, but haven't tested yet. This is all experiments I've been doing on my own, there's nothing ready to be released.
That quote is referring to the mechanism for backing up passkeys that only exist on a hardware security key, which deliberately does not exist. Moving keys onto a security key is possible for some devices, but that is a deliberate compromise, as these devices are designed to generate their own keypairs and never leak secret key material (thus preventing backups).
Ops, I may have misinterpreted. I read it as take a passkey and copy/store it inside a hw sec key. Thanks for clarifying!