That's literally part of what makes a passkey a passkey (v.s. just a WebAuthn credential), so that's a given.
> as insecure as a password
No. Passkeys can't be phished, passwords can. Passkeys can't be cracked after a data breach. Passwords can. Passkeys can't be set to something easily guessable. Passwords can. Passkeys can't be written on a post-it note and taped to your monitor. Passwords can. Passkeys can't be reused across multiple sites. Passwords can.
There are so many ways passkeys are superior to user-memorized passwords from a security perspective, it's laughable to call them "as insecure as a password".
> One could allow multiple instances of authorized passkeys, but those would be even more insecure than passwords, because malicious software on your device could create evil new key instances.
What? Malware stealing your password is "more secure" than malware registering it's own malicious key to each individual site it wants access to?