Oh great. So a physical security key could be a passkey, but only if it's used as 1FA not 2FA. At the same time, this weird "store key material in a device and optionally sync to the cloud" thing is called passkey, but if someone uses it as 2FA it suddenly isn't a passkey?
The other thing is that it is more convenient to have "discoverable credentials" (a.k.a. "resident keys") so the browser can check if there are passkeys stored on your FIDO2-authenticator for a specific website. (instead of asking for a username first)
Syncing is optional but if your passkey is not synced, it is usually called a "device-bound passkey" (e.g. Windows Hello).