I think their point is that they don't want to have the window of vulnerability (to loss) when they have added a new account but haven't yet rotated their off-site key?

That said, the real answer is that FIDO keys can be synced by e.g. Apple (as described in more detail here: https://www.wired.com/story/fido-alliance-ios-android-passwo...). So you can potentially just make your offsite backup be a hardware key that gets you into your iCloud keychain, and (if you are willing to trust Apple) use your iCloud for backing up all your other accounts' keys.