The article specifically discusses auth via app, but if it's involving the FIDO alliance, it'd be weird to exclude hardware keys, I guess. I still don't like the idea of going single factor, but if it's with a hardware key, I can see it being better than with an app since it has to directly interact with the process itself.
But, of course, if this is optional, I still have to reference the end users. I'm willing to pay for an authentic FIDO key, which can be a tad costly. Your typical user might be more inclined to go for a cheap one that does enough to get into the account, and may not be trustworthy, or would prefer not to do it at all.
My understanding is that the theoretical app being discussed behaves in the same way as a hardware key - it is simply a software-only implementation of the protocol (and thus comes with the same advantages).
For me the question is if this is a webauthn thing in general or a security key thing (to include the domain in the challenge to prevent phishing)