Ultimately what he is suggesting to do is to bind a string of entropy to a website through facilitation of the browser and a Passkey.
A cryptographic seed is one of the most sensitive things. And here you choose to expose it to a website (even though it was specifically generated for that website). This is not something you do for authentication. The only reason to do this is to have javascript/wasm on a website perform sensitive cryptographic operations for you. You should never be doing this.
You can run the PassSeed code/mechanism on your own domain or localhost to ensure it's not subject to malicious host exfiltratuon. I agree that one should only trust a foreign host with low-security uses under this scheme.
A cryptographic seed is one of the most sensitive things. And here you choose to expose it to a website (even though it was specifically generated for that website). This is not something you do for authentication. The only reason to do this is to have javascript/wasm on a website perform sensitive cryptographic operations for you. You should never be doing this.
Applications such as password managers can already integrate entropy from a passkey to encrypt their databases using the Challenge-Response protocol: https://docs.yubico.com/yesdk/users-manual/application-otp/c...