I know, I know (or that is what I thought).
That's why I started this thread saying "contradiction".
Look at this exercise, from an electronics book.
It is trying to illustrate and be an example of the kind of exercise I attached at the top of the thread, applied to a shunt-shunt negative feedback.
The input resistor is 1k.
The answer (at the bottom) states that feedback circuit input impedance (Rent) is 150 and not 1k. It happends to be almost Rin / (1+AB). Same goes for output impedance (92 ohm - look at not below).
( -870 V/V is, as you can guess, voltage gain).
Note: opAmp input impedance is not supposed infinite, but 100k (Rid). Output, 1k.