So who do you think taught him his technique?
Horowitz was writing his autobiography with Harold C. Schoenberg, who tape recorded many conversations they had while doing this project: it was never finished, Horowitz started some recording projects, and then he was gone. Schoenberg published a book about Horowitz using these tconversations as a groundwork; fascinating stuff, he said things he had never said before... a must read if you are into Horowitz, I unfortunately do not have it, but I re member reading the following in it.
He said that he had been a terrible student, he never practiced his scales or exercises until examination time and drove his teachers (pre-Blumenfeld) crazy with his erratic practice habits and screwy hand position, etc. He then went on to say that artists have to figure these things out for themselves, or something to that effect. Don't remember exactly...
The reason for the flat handed position he frequently (not always) employed had to do with his fingers being double jointed; double jointed people either cannot or don't have to keep their fingers curved in passagework, not sure why as I am not double jointed; I had a student who was, and her playing was fine; she came to me from a teacher who wouldn't teach her anymore because her fingers wouldn't stay curved

. It didn't hinder her, and it certainly didn't hinder Horowitz. I've also read that Horowitz observed Blumenfeld playing flat fingered owing to the effects of syphilis (sp?) and that he imitated this, and that this was the secret of his sound, if not his dexterity.
Horowitz himself said that his technique was not extraordinary, but that his ability to control dynamics and produce many different sounds from the piano was what made him unique. I agree with him; yes his digital dexterity (particularly octaves

) was extraordinary but other pianists such as Cziffra, Hofmann, Michelangeli and Hamelin have a cleaner and more precise digital and physical mechanism than he did. I heard him live several times, and the sounds he made come out of the piano were unbelievable, as his recordings bear out. There was also something else, that undefinable thing, when he sat down at the piano, this strange almost supernatural energy would fill the hall, it would have been terrifying if it was not so beautiful. Watch this....
https://www.youtube.com/watch?v=XebJY_y4zrE&search=scriabin