Well everyone learns differently. My earliest "compositions" (if you can call them that) are from grade school whistling into an old tape recorder and then transcribing them on our upright piano. Many of my friends who are musicians could also compose based on their ability to sing LONG before they had any working knowledge of music theory.

Additionally let me call out that sight reading and composition are completely different objectives.

I also sincerely doubt playing random notes on the piano as part of a sight reading process would make you a better composer unless you're aiming to be the next Karlheinz Stockhausen.