Right, but the purpose of key pinning is to prevent a MitM from learning the plaintext, not Alice or Bob. There are any number of means an attacker could employ

Right, but the purpose of key pinning is to prevent a MitM from learning the plaintext, not Alice or Bob. There are any number of means an attacker could employ to reverse-engineer an API (decompilation, memory dumps, debuggers, ...). This is asking for more of a DRM-like solution (with all the issues that come with it).