I do not believe this is true. Passkey is only a 1 user, 1 key system. This means that to login on a second device, the 1. key must be exported from the first device, 2. encrypted, and 3. transferred to the second device. Each device needs a copy of the private key.
You can add multiple passkeys on your Google account today. If a website is dumb enough that they only allow a single passkey per account, they have misunderstood passkeys profoundly.
Google != passkeys. Google may allow multiple keys, but that is outside of what FIDO specifies for passkeys.
Here's a source saying that passkeys expect one and only one key:
"Passkeys are designed to [...] allow the FIDO credential to roam across multiple devices. This [means that there's ] no need to repeat enrollment on each device [.]"
> If a website is dumb enough that they only allow a single passkey per account
