The likelihood of a US ban preventing ICBM development when the basics are inherent and already known?
Nothing in the story really related to this risk. But, you are probably right it's the latent fear. The ban only drives Iran closer to Russia. Why buy US tech when you can cut out the middleman and go to the same source the US does for engines? (I know they don't buy much any more but they did for a while, and presumably the Iranians can operate a Russian engine as well as the Americans can)
The ban is about delay, not the absolute of prevention. That much is working. Iran is having to develop technology locally as opposed to simply importing it. Without any restrictions, Iran would simply purchase ICBMs and deploy them in a matter of weeks.
A delay would be logical if it provided time to develop a countermeasure. But other than point-defence ABM systems there is no universal countermeasure, practical or in development.
The delay isn't about just about technical countermeasures. It's about giving time for other options to work. These could have included: engagement and diplomacy, regime change, or direct action.
In addition, by increasing the cost of developing these systems, you force hostile (or non-friendly) governments to split their resources. Every bit of funding that went into building a rocket is a bit of funding that doesn't go to Quds Force.
I believe the game plan is to starve the population during the delay, fostering dissent (or just chaos) so a puppet government will be in place before the icbms.
Because that went so well the last time, after all.
All the powers that be might be happy to step into the gaps left by the others, but I'm not sure that automatically means they throw them ballistic missile tech outright. That would also not be in the Russian's interest to some extent too.
Nothing in the story really related to this risk. But, you are probably right it's the latent fear. The ban only drives Iran closer to Russia. Why buy US tech when you can cut out the middleman and go to the same source the US does for engines? (I know they don't buy much any more but they did for a while, and presumably the Iranians can operate a Russian engine as well as the Americans can)