I think he's wrong regarding perfect pitch. Relative pitch would be enough to be able to "auditorize" (like "visualize" but for sound) works in his mind.
Having studied music from an early age when as he says his hearing was superlative, he would have internalized the timbre of each instrument through its playable range, and could use that knowledge to choose what (absolute) pitch to choose for various passages. He would not have to "hear" the absolute pitch in his head; melody and harmony are invariant to transposition.
That said, he may have had perfect pitch, but that in itself would not enable him to hear a complex orchestral piece in his head.
I wonder whether absolute pitch in the sense we consider it today is even applicable to Mozart’s time. Pitch standards are a creature of the 19th century, and before their adoption, pitch varied considerably between cities, orchestras and even buildings.
In that context the timbre of pretty much every non-keyboard instrument would be the only real reference point for how a melody might sound - a bassoon from a maker in Vienna may have been a quarter-tone sharper than one made in Dresden, but they would have had a recognisably similar tone colour when playing eg an E flat.
That’s not to say a freakishly good ear might not have been able to distinguish a Dresden pitch from a Viennese - but as you say, that’s not material to composition. Knowing whether that that E flat will sound confident or mournful would be way more relevant to how it’s used in a phrase and ultimately what key to set the piece in.
I agree the absolute pitch that orchestras played at may not have been consistent like it is today. However, perfect pitch is not really about Western standards or whether A equals 440. It's the ability to remember a pitch in absolute terms.
As a musician, I have a finely-tuned sense of relative pitch. I can pick out chords and melodies easily, but I don't know the reference key unless I hear an instrument and I'm told what note it is playing.
My son has perfect pitch. This became apparent when we watched the movie "A Hard Day's Night", and he complained that it was out of tune with respect to the Beatles albums he had heard.
I listened carefully to the two sources, and it turned out the movie was about a quarter tone flat compared to the recordings. Only a person with perfect pitch would notice that. The rest of us forget the absolute value of a pitch in a matter of minutes or less.
I can imagine complex orchestral works and I'm no musician. It must be incredibly common and orthogonal to absolute pitch, because imagining music is a prerequisite for playing an instrument, and most people can play at least air drums.
[0] https://www.youtube.com/watch?v=3bA2V0jZMo4