Ha… I can add, that those “guardrails” can already be circumvented here.
If you choose the “Phone” route here, you can store the passkey in the Bitwarden vault (at least on my Android 15 device I was able to store the passkey via the BW mobile app).
But it comes with some caveats: besides the possibility of “circular dependency”, BW can’t store it’s own login-passkeys “with encryption”, as the BW vault itself can’t store passkeys “with PRF”. (see this feature request: Support for Storing PRF-Capable Passkeys in Bitwarden Vault)