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.