So maybe I don't get it, but I always understood that 2FA means something you know and something physical you have. Now if I can get they keychain using something I know, does that not somewhat defeat the purpose of 2FA?
In general it's "who you are" (biometrics) as well as "what you have", with the OS being the one ensuring that the phone itself was unlocked and having an extra biometric check when signing in with passkeys; this is how iOS currently works, it pops up face ID before it signs any Webauthn challenges.
Also, ideally, your syncing passkey solution (whether that be 1password or iCloud Keychain) would itself be a combination of multiple factors before you can get in - in the case of iCloud Keychain, 2fa is on by default on your Apple account, and the keychain is also protected by your password plus the passcode of one of your devices. In general this is already immensely more secure than passwords because the website is verifying a signature instead of the correctness of a shared secret. So, it'd still be possible to have 2fa with the first factor being passkey and the second factor perhaps being another physical security key or maybe verification of an email code, but that would likely be reserved to enterprises and high-security applications.
(I assume Apple themselves aren't going passwordless themselves anytime soon, especially with how that'd work on fresh devices).
Typically MFA is something you have (physical possession), along with something you know (secret) or something you are (biometric).
This is more abstract than physical possession of a single device with a non-exfiltratable private key. There are synchronization processes (so its one of many physical devices, on a sync fabric which allows devices to be added).
The process for adding a device should require multiple factors as well, but I believe there ultimately is a typically a recovery mechanism like a printed recovery key which would make this considered single-factor.
However, most deployed 2FA is via SMS, email, or backed-up TOTP today. The goal is to build a much more secure system that is recoverable enough to get consumer adoption, not to try to achieve say NIST 800-63 AAL3.
One ongoing proposal is that you get an additional device-bound factor as well. Seeing a new device-bound factor would let you decide to do additional user verification checks if desired.
