Perhaps they should’ve said platforms. Because if you wanted to migrate those passkeys off your password manager and into a different platform like Apple Pass or Google how is that accomplished?
Once 1Password supports proper single export when I make a new passkey I'll store it there and later export it to Apple.
Meanwhile I simply make two passkeys. I've only run into I think two sites that supported passkeys but would not let me make two.
On most sites making a second passkey is as simply as going to your security settings, finding the passkey settings there, hitting the "add another passkey" link, and pointing your phone at the QR code it shows, and then on those phone choosing the password manager that you did not use for the first passkey.