Why not? Parent raises a good point, how do we define a single task?

If you are playing one melody with your left hand on the piano and one with your right, is that one task or two?

Because everything but the executive task flow is being handled via processes known as automaticity.

https://en.wikipedia.org/wiki/Automaticity

We don't have a definitive diagram of the mind to point to, but I feel we all really can feel what is and what isn't multitasking.

Once internalized through experience, hand movements on piano (or computer) keyboard are subconscious. They don't tax your brain. Thinking about what you're playing/writing and why are you doing it is still within one "context of execution". They are closely related mental activities sharing most of the same data. Starting to simultaneously think about or engage in an unrelated, cognitively demanding activity - this is multitasking, or the so-called "context switch".