[0]: https://www.amazon.com/Countdown-Zero-Day-Stuxnet-Digital/dp...
What keeps this cooperation secret? It would only take one weak link at any one of those companies to reveal -- accidentally or otherwise -- that they were coerced into providing their signing keys. As soon as that got out, speculation runs amok: Are all products from said company compromised? This would be ruinous to a company, so no one in charge would agree to that without something significant -- which would be even harder to hide from the public -- in return. Then, who asked for the key and why? Could that be traced back to the (presumably) agency in question? That weak link was weak once, there's nothing to assume that he/she won't be weak again, etc., etc.
The sophistication of this software is that it did all this undetected.
Everything's an API at the end of the day.
There was an article on here recently about the practical effects of the TLS 1.3 drafts, and how they broke proxies. I can't find it. It was talking about how the proxies choked because so many variable wire bits in TLS 1.2 stayed static for so long they rusted in place in proprietary implementations, and the proxies crashed once the values changed (despite this being a concrete protocol violation). This situation reminds me of that.
The author sensationalizes the effort of the creators, painting a Holywoodesque scenario where they break into every possible software company to steal keys to misrepresent the software, going undetected by every possible security company etc. Since this is a Quora post, I can live with him playing to the gallery.
Given the amount of speculation of involvement of US and Israeli intelligence agencies, and the task this worm was assigned, the real effort might have been just about writing a USB worm that identifies specific machines and handing the USB-0 to a double agent (I stopped watching Homeland after season 5 and am rusty with the jargon). The rest of it would have been simply asking all the associated software and hardware companies, politely, to cooperate.
If any of this is true, stuxnet is anything but sophisticated. Its just lots and lots of specific API calls.
That brings up the question: what is sophistication as applied to software?