It is specifically about the situation when you are gonna drive the car and A/T is in N position. The manual cautions that in such situations accelerator pedal shouldn't be pressed because shifting from N to D will cause sudden acceleration leading to an accident. Shifting from P obviously can't be done without pressing the break so can't understand why they have mentioned P. This is obviously not regarding down or upshifting of A/T lever i.e D to 3,2, L or vice versa.
For downshifting the A/T lever i.e D to 3,2,L there are speed limits defined in the manual for each lever position. It doesn't matter whether accelerator pedal is pressed or not, one can down shift to the desired lever position if car is in the corresponding speed bracket. For upshifting there are no restrictions of any type i.e speed bracket or accelerator pedal pressing or not pressing.