But webauthn.io and webauthn.guide are both copyright of Duo Labs and so is the library so I guess they can do whatever they want ;) I assume the two websites and the library are by the same author
Also the library lives on the server side ; not client ; and given the license is not AGPL there is no need to state that they're using it either.
(Edit: original comment was accusing me of copyright infringement for some reason)
EDIT: Disregard the rest of what I wrote here previously, I didn't realize it was Duo Labs's own website.