Well, of course they are. American Green Berets ran an extensive "train the trainer" programme in Afghanistan, bootstrapping the precursors of the Taliban into a military that could take on the Soviets.
Then once the Soviets were gone, the factions turned on each other, which meant, they continued to practice and refine what they have learned. An ISIS equivalent of a Drill Sergeant probably isn't too different from his Western counterpart...
The real origin of ISIS's organizational skill is the disbanding of the Iraqi military in 2003 - a totally unnecessary decision on the part of the US that played a big part in the ensuing chaos in the country. Lots of military officers from the old regime, disproportionately Sunni, out of a job, and none too picky about the specific ideologies of the groups that would advance their sectarian interests, transferred their knowledge to groups like AQI, the ancestor of ISIS.
So not only the military knowledge, but the people that know how to run the factories and the people that can handle the logistics of such a thing. Not to mention teachers, medical staff, and similar posts.
It's not obvious exactly why the IS is different, but I doubt it has anything to do with American influence. Other rebel groups in the Syrian civil war has had direct American training but they were highly undisciplined and ineffective.
Lots of people have big ideas. Few can competently execute on them. This is true of commercial entrepreneurship and international terrorism alike. From what I can tell, ISIS is (or was) unusually well run [1].
[1] http://m.spiegel.de/international/world/islamic-state-files-...
I think one of the reasons that 'these people' aren't frequently over-estimated (or even accurately estimated) is because almost none of them have higher education from their own countries.
Not that many higher education facilities have a specific 'learning from your enemy' track, and it's not as though there are respected rules around not using your enemy's facilities against them ... but you know what I mean.
