I'm not sure I follow you. ADB was ahead of its time, and a precursor to USB, but nobody's claiming it was an open protocol. What was important from an electrical engineer's POV was that the connector could be replaced using off-the-shelf parts. That was because the connector was standard. If you wanted to develop for Apple, you had to own a company and strike a deal with Apple. Not so easy, but certainly possible.
Now, if you just wanted to hack peripheral hardware at the time, you would have invested in a lot cheaper (expendable) computer with a standard serial / parallel port, so I don't really understand what you are saying.
That's what API design is so important, and so difficult. By analogy the DIN connector is simply calling convention and ADB the actual function calls.