The problem with passkeys is that they're essentially a halfway house to a password manager, but tied to a specific platform in ways that aren't obvious to a user at all, and liable to easily leave them unable to access of their accounts.

Agreed, in its current state I wouldn‘t teach someone less technically inclined to solely rely on passkeys saved by the default platform if you plan on using different devices, it just leads to trouble.

If you're going to teach someone how to deal with all of this, and all the potential pitfalls that might lock them out of your service, you almost might as well teach them how to use a cross-platform password manager

Using a password manager is still the solution. Pick one where your passkeys can be safed and most of the authors problems are solved.

The only thing that remains is how to log in if you are not on a device you own (and don’t have the password manager). The author mentions it: the QR code approach for cross device sign in. I don’t think it’s cumbersome, i think it’s actually a great and foolproof way to sign in. I have yet to find a website which implements it though (Edit: Might be my specific setup‘s fault).