What is the upside down experiment meant to prove? Seems to me that the mental rotation issues indicate that mental image processing is not very rotation toler

What is the upside down experiment meant to prove? Seems to me that the mental rotation issues indicate that mental image processing is not very rotation tolerant, but rather needs a hardwired (and slow) counter-rotation step added to cope with rotated symbols, which you could just as well tack onto a neural network. Am I missing the point? Also, being able to consciously recognize letters is relatively easy, but the normal reading process, with which people recognize well-known letters and instantly unconsciously convert them to sounds, does require quite a bit of repetition of those letters before it starts to kick in...