He made liberal use of the una corde pedal and so kept his left foot on it in passages marked ppp through mf, but once he reached f-fff, he pulled his left foot under the bench to give him more leverage in the loud passages. In this YouTube video
, go about 16 minutes in and 40 minutes in to see this. He actually lifts himself off the bench slightly when he wants maximum power.
Horowitz played idiosyncratically, like many great pianists. He figured what worked for him to achieve the sound he wanted. The Steinway he played was regulated to his specifications and supposedly had a very light touch and very hard hammers, so it makes sense that he utilized the una corde quite a bit.
When I was young, I was taught to keep my feet off the pedals unless in use. In particular I was told to avoid using the una corde pedal in order to learn to play p-pp by controlling my fingers and arm weight to achieve these dynamics. My teachers postponed teaching proper pedal technique until I was just about out of the beginners stage.
Now, youth long gone, I keep my right foot close to the sustaining pedal but usually keep my left foot in a neutral position several inches from the una corde. I can balance on the bench better that way. If I need the una corde, my left foot can always find it. If I need power for ff-fff, I can move my left foot under the bench.