I doubt it and it doesn't matter if he could or not, at least IMHO.
His job was to find the people to build the next great idea he had, not necessarily to do it himself. At least from reading the book written about him it seems that was his process in general. Not that I have any first hand knowledge of that, just what I read.
It gets to the heart of the tech-entrepreneur dichotomy
quite well. My initial thought was well, yes, take three months off and he could easily do a decent app - but the opportunity cost would be enormous
But this just assumes there is a single coding based dimension (the Blub dimension?) - in reality Jobs stopped coding in Objective-C a long long time ago, and started coding in vision/people/organisations - a much more difficult programming environment and one that gets really really hard to scale.
I can barely program in this language on 6 or more cores. Jobs managed it at 100,000 cores.
So my answer is no he could not write an iOS app - but he could write really good applications in HomoSapien (Enterprise Version) - and there is hardly anyone who can get that to even compile.
His job was to find the people to build the next great idea he had, not necessarily to do it himself. At least from reading the book written about him it seems that was his process in general. Not that I have any first hand knowledge of that, just what I read.