I think based on the parent posts choice to go with the "who is your customer" definition and the fact that we intuitively think of RabbitMQ as some sort of systems program, it probably fits the modern definition of systems program that we all kind of agree with but do not necessarily put into words.

I don't think RabbitMQ being written in a "non systems language" is relevant. Its a systems program even if it was written in shell (if it was it might be a bad systems program, but a systems program nonetheless because of it's intended customers and use case).