How did Mozart have absolute pitch?

Equal temperament was already quite well established by Mozart's time (remember Bach's 'Well-Tempered Klavier'?). And 'absolute pitch' is only a good memory for pitches - so Mozart merely 'remembered' the pitches he was exposed to as a child. Whether 'A' was at 415, 430 or 440 Hz doesn't matter - it's what HE remembered 'A' as that matters.