It’s actually the opposite, instinctive finishing is very difficult (if not impossible in most cases) to teach—it can improve slowly over time with match experience but, at this level, you either have the natural mental acuity or you don’t. And luckily Raheem has it.
‘Composure finishing’, on the other hand, is something that develops over time through a combination of experience, coaching, and body development (the last being needed to physically execute what the player’s mind is envisioning, which is difficult as a child, but becomes easier as you become an adult, before becoming more difficult again as you age out of your “prime” physical years).
For all his recent misses, you can actually see that Raheem has improved the latter substantially in recent years. And there is no reason to believe that under Pep team tutelage he won’t continue to develop that skill. After all, he is still 3-4 years off the normal peak for his position, at which point he could inhabit that lofty perch for 2-3 years, potentially giving him (and hopefully us) 5-7 years of 25-40 goals per season output.
Forget his football intelligence, pace, movement, tenacity, and hardened mentality— the fact that he could really improve his composure should be absolutely frightening to any opposition defenders and managers.