I think your criticism of the term is somewhat valid, in the end these are just back-ends with APIs :)

However, the point the platforms are trying to make is that their APIs have been written with the explicit purpose of building a frontend ontop of them later on.

A lot of the old-school platforms have some APIs as well, but they are meant to e.g. import products or export orders. They are not meant to implement a frontend on top of, because they have a frontend engine as part of their monolith, and many functions you need to write a proper frontend will not be available through their APIs.

So the term "headless" means "we have APIs, and they are designed for you to build a frontend on top of".

Thank you. The only usage of that term that I was familiar with was that given here:

https://www.howtogeek.com/660841/what-is-a-headless-server/

A “headless” computer system is just one without a local interface. There’s no monitor (“head”) plugged into it. There’s also no keyboard, mouse, touchscreen, or other local interface for controlling it.

Well said!

MVC frameworks like Django, Rails, etc, are not headless by default, as they take care of all frontend rendering.

Headless = frontend and backend are fully separated; the "promise" was to have the backend be used on multiple projects. You would have a headless backend, then a web frontend project, an iOS one, an Android one, etc without having to rewrite any backend code.

So "headless" = " Web API"