The pianist I mentioned is primarily known as a composer. In the classical music world, we use the term realization for the performing of a lead sheet, or originally, figured-bass notation. Realization, improvisation, composition, they freely interacted in the performance of this dude. Like a composition built in real-time from the needs of the moment. I wouldn’t be surprised if he transposed his accompaniments up or down, during the rehearsal process, to accommodate the range of the singers. And there’s no button for that on the piano.
Knowing music theory allows you to comprehend musical abstractions. When we use language, words carry the meaning of abstractions. We are able to have certain thoughts because we understand the meaning of words.
No, you don’t need any of that to arrange random sounds on a quantized grid.