It's the combination of feedback, decision making and then successful completion of the challenge that really combines to create the feeling.
Thinking of the problem, planning your response, and then executing in quick succession repeatedly.
Especially if it's off-piste but an area that you're familiar with. You plan out your run multiple steps before you start, and once you're on the hill you have to be flexible enough to change course and replan on the fly for another multiple steps ahead, all while you're barreling down a hill.
And then there's the moments of sheer terror when you see snow sliding past you, or when you come up on a recent avalanche 1) have to avoid the packed ice and 2) look out for what caused the avalanche.
You're absolutely right - I get a kind of rush from coding that's similar to skiing off-piste. Not quite the same but not as different as one would think.
I do think that the author doesn't understand Flow fully. The ideal state of Flow is when there is a concrete challenge with immediate feedback, which coding is great at providing. The other activities he compares coding to don't have such strong feedback or concrete challenge.
The only activity that compares with coding for me is rock climbing. While it is a physical activity, it has the same kind of concrete challenge and immediate feedback. It is totally immersive.