Passkeys can be transferrable and syncable ( see keepassx, bitwarden, and relatives). But ideally, eventually, you never sync them because every time you log into a service with a new device, that device gets its own passkey. No need to sync them and you can revoke a devices access if it is lost or stolen without effecting any other device. They are closer to client certs than passwords in that respect.

FWIW, the article may be a bit outdated. I'm not a FIDO expert by any means, but there have been improvements in the spec to help define how to transfer passkeys (CXF and CXP). The situation should be better than 2024, but if you're using TPM-locked passkeys, you're just in that locked box already.

I think there's somewhat of a conflation of concerns. It sounds like your concern is, I have a credential manager and want to migrate to a new credential manager. I just want it to be simple to migrate my massive number of passwords, right? I need to bulk export + bulk import. EASY. But... what if your first password manager has a bug in the seed generator? Oops, you have to rotate 500 passwords now... there is no easy, off the shelf solution for that. It's basically the same for hardware-bound passkeys. If you generate hardware-bound passkeys, you're not going to simply replicate them to another hardware-bound passkey. The whole purpose of that type of credential, is to make it hardware-bound. There are a lot of meaningful reasons to do that from a security POV.

Now, I think what probably needs to be made clearer, is that if you do want that kind of credential, you shouldn't be using TPM-backed passkeys. You should be purchasing independent hardware (FIDO2 hardware keys) or only make software+transferable passkeys. Personally, I have several hardware keys and I register them with one being a backup. If the vendor posts a vulnerability notice that there's some big gap in their hardware, I will have to go and rotate every passkey. I know that is what I am signing up for, but maybe not a lot of people do.

I haven't found this to be the case in practice (IE passkeys are plenty portable). I've successfully used passkeys on multiple browsers, OS's, and apps, all stored using a FOSS password manager (keepass). Any FOSS password manager that is either self-hosted, or syncs, should work just as well with passkeys as it does for regular passwords or TOTP.