I don't think Horowitz played with flat fingers. He used to hold his non-playing fingers straightened out, and this fact, coupled with his very long fingers and extraordinarily elastic last (DIP) finger joints creates wrong impression of playing with flat fingers, but it's not the case really. If you watch closely the playing fingers in the octaves that you refer to on the video, you will see that his middle (PIP) finger joints do bend downward when he touches the keys.
Besides, talking about Horowitz we shouldn't forget that technically he had serious restrictions, e.g. he could play comfortably only on his bright grand with extremely light action, he had to sit uncomfortably low, and even in these conditions his technique was very unstable, many of his concerts were a complete failure technically. This led him to an extreme stage fright and a long break in the middle of his career. So if someone wants to find a pianist to learn technique from, I think Horowitz is not at all a good candidate.
I don't think many of Horowitz's concerts were a "complete failure technically" except possibly when he was heavily medicated.
Obviously I had no chance to visit his concerts, I was only 6 when he played in Moscow last time, so I can only tell you what I read. I read that he had concerts which were a technical failure and it led him to an extreme stage fright and the break, and it was long before Japan.
They certainly were not a failure in his prime and much of his reputation was built on his phenomenal technique.
I didn't say that he hadn't brilliant technique. I said his technique was unstable, that is, he had technique fluctuations from concert to concert that were much bigger than what is considered normal for elite pianist, and these fluctuations did not let him play well consistently.
Preferring a piano that's bright and has a light action doesn't mean one's technique is faulty.
I think there is a difference between preferring a piano and refusing to play on all other (well prepared, elite brand) pianos except his own. I consider it a technical drawback.
Hoisting a piano out of a window of a New York apartment by crane and transporting it to Moscow seems a little bit over the top, isn't it?
I also can't agree with your claim that Horowitz didn't play with flat fingers. He has been mentioned for decades by countless people as the prime example of a flat fingered pianist.
I'm sorry to be contradicting countless people, but I say what I really think. I think Horowitz had fundamentally pretty normal technique, he relied on arm weight as well as others. Most of the time he played with curved fingers. In many, many passages he crooked his fingers even more than others in average. He used to flatten his fingers only in some cases when it is appropriate, like playing cantabile, and when other pianists do it, too. He didn't play his famous octaves (or anything else unusual) with flat fingers. It's only his low posture, the anatomy of his hands and his habit to straighten non-playing fingers that created wrong impression. He was not a 'flat fingered pianist'.
And certainly he didn't play with flat fingers in a manner that we often see on YouTube (i.e. with a collapsed hand) and what people here might think when they hear 'flat fingers'. That's why I am a little bit worried every time when Horowitz is mentioned in that context.
You seem to say that his non-playing fingers were flat while his playing fingers were curved but when one curves one finger the adjacent ones curve automatically.
They curve automatically unless you deliberately resist it, and this is exactly what I think is happening. The uplift of his last (DIP) finger joints that we can often see on his recordings shows that he simultaneously
uses his finger extensor muscles in the forearm with finger flexors that flex middle (PIP) joint. So he simultaneously flexes PIP joint and extends DIP joint, and this I think is a hallmark of his technique.
If you're interested, look at 0:08 of Animisha's video. You can clearly see at slow motion that his 5th finger is bent when playing the key while his 3rd and 4th fingers are straightened. Then look carefully at downward passage at slow motion. You can see that he bends his PIP joints while his last phalanges are still lifted. That I think is the secret. He deliberately tries to uplift his fingertip when he bends PIP joint to make a fingertip contact the key with larger surface. But unfortunately it is completely impossible if you don't have his unique DIP joints anatomy, it just can't be copied.