I completely disagree with that. Pretty much everything involved in goalscoring involves a coachable trait. You can coach players to find space, you can coach one-touch finishing, you can coach players to hit the corner of the goal by constant training of that skill, you can coach a player to be more aware of when and where to make their movements, you can coach an attacking unit to have patterns of play to open up goalscoring opportunities (it's true, other teams actively do this). Composure comes with consistent and regular successful repetitions of a positive action, in this case kicking a ball in a net past a goalkeeper.
You can work on mental aspects with a player, you can work on their confidence, help them understand where they need to be - one of my favourite ones at the moment is with Unai Emery and Ollie Watkins, as apparently all Emery has done is tell Watkins not to spend too much time running out wide and instead stay in the middle where he can score goals. Hardly rocket science, but Emery is hailed as a genius for doing it. I mean, he IS a genius, but not for that. But it's an easy example of coaching a player to get more out of him in a goalscoring sense.
Sure, you still get what I suppose you could term a 'natural' goalscorer who will need less work on these things, but to suggest you can't improve a player's ability to score a goal through coaching just isn't true.