No one knew before Donnarumma proved it, but one thing is certain and must be acknowledged: Donnarumma is a great goalkeeper, and he has proven it, especially since his young age allows him to quickly adapt to the playing style implemented by Pep Guardiola.
I find Manchester City's early season experience a bit confusing. Despite having a strong squad across all aspects and a great coach like Pep, their performance hasn't improved. So, there are things beyond technical aspects that Pep needs to improve, perhaps tactics or player chemistry.
You made great points about Donnarumma and Manchester City. You are right that Donnarumma is top goalkeeper and his youth and ability to adapt quickly would make him perfect fit for coach Pep Guardiola. When it comes to Manchester City it is frustrating to see them struggle despite having great team and world class coach. Problem likely is not just about their skills. I think their can be low team chemistry or issues with their tactics. Pep will have to figure out what is wrong to get them back on track.