We had originally planned to go all-in on passkeys for ONCE/Campfire, and we built the early authentication system entirely around that. It was not a simple setup! Handling passkeys properly is surprisingly complicated on the backend, but we got it done. Unfortunately, the user experience kinda sucked, so we ended up ripping it all out...
His whole premise is undermined by him not doing any research on the topic before deciding to write a blog post. Proton passkeys for instance, are cross platform, and the ability to transfer passkeys between devices is one of the features being worked on by the other providers.