is the author suggesting allowing the client to set their own claims and using that to auth whatever action they are going to take? I have to be misunderstanding what they are saying - that sounds fraught with risk
But this scheme is flexible. You could also have the client send "requested" claims for the server to consider adding if allowed when getting a JWT.
You could also reverse-proxy client requests through your server, adding any claims the server allows.
In that case, the client can possess the JWK keypair and do its own signing.