One of the Co-Authors here

There are two elements here. Agent can start a full authorization request with AS through authorization code grant flow, even requiring a step-up or some rich authorization details, therefore whatever OTP by SMS or Magic link is an AS - Subject/Client "problem".

For Agent that cannot start a full authorization request (too costly, to complex, subject directly unreachable at the moment), we have a mention to OpenID Connect CIBA into it. With it, the Agent will start a back channel authorization request with the AS and the AS will use a method of authentication / confirmation with the subject in front channel, for example sending a SMS or sending a link to click. Again the resolution will remain an AS - Subject/Client "problem".