Thanks for the long answer and correcting the mistakes I made in my question! (Edited my question to save readears the time)
Camera matrices are called K, not A
They called the Camera Matrix A all along the the documentation that I linked to. I tried to adapt 
The Main point of the discussion and my question is:
that quantity does not exist. it cannot be obtained for math reasons. that information doesn’t exist in reality, and not in theory.
And if you’re so sure about that, I’ll trust you. I’d still be interested in a detailed mathematical explanation of why.
So this stackexchange answer is definetly completely wrong?
What you’re saying is that even though i have a calibrated camera and a z coordinate to my object it is impossible to figure out the size of that object because the camera matrix can not provide that mapping?