I believe you are confusing two issues. The question was if an AFCI receptacle can be used to protect the branch circuit wiring if the branch circuit contains a shared neutral. My answer is that _if_ the AFCI receptacle contains ground fault detection as part of its AFCI function, then the shared neutral will be a problem.
This is not a case of a system that requires GFCI protection, but rather the issue caused by the 'implied' ground fault protection in some AFCI systems. My understanding is that AFCI systems do not require ground fault detection, but that early AFCI systems used ground fault detection in order to meet some of the performance requirements.
If GFCI protection is required for this installation, and there is a shared neutral, then as you say the only choices are separate protection at each receptacle or the use of a breaker. A receptacle device cannot be used for ground fault protection of a shared neutral circuit. (In theory, you could have a receptacle type device that monitors both legs of the circuit, but I don't believe these are manufactured.)