The concept of a passkey as desired by Yubico was that every user buys a set of hardware keys, uses those instead of passwords, and has no ability to authenticate otherwise.
The concept of a passkey as desired by Apple, Google, and Microsoft is that every user magically authenticates using their OS, and has no ability to authenticate otherwise.
The reason the UX is confusing is because the OS vendors don't want the users using non-OS software or hardware - they want you to use a cloud-hosted passkey instead of using a Discoverable Credential on a Hardware Authenticator, and instead of using a password manager providing its own sync facility. This is shown in the screenshots in the article.
The ideal future state is:
* the provider for newly registered credentials would be a browser setting
* the setting would come configured to use the OS vendor out of the box
* installing a password manager providing passkeys would prompt to change the setting to use the password manager instead
* one of the options in this setting would be "prompt me every time". Approximately nobody would choose that option
* there would never be a prompt for what to use on authenticate() calls, only register(). Authenticate would use whichever valid credential you provided first, whether that's plugging in a token or scanning your thumb to unlock a TPM or whatever
In this world, 99% of people are using OS-vendor-provided cloud-synced passkeys, but technical users get what they want too and everybody has both a secure and an easy experience.
The thing stopping us from getting to that ideal state is that the provider of the FIDO "platform" (the software that lets you choose a key to use) is the OS vendor instead of the browser vendor, and they have a conflict of interest because the OS vendors are also cloud services vendors.