I think their goal is to have transport layer be a pluggable subsystem, divorced from their request-response protocol. They even essentially say that HTTP is n

I think their goal is to have transport layer be a pluggable subsystem, divorced from their request-response protocol. They even essentially say that HTTP is not necessary to their protocol, so I think it's pretty apparent that HTTP is a utilitarian choice, but they're not trying to build email-over-http.