[pjc50]

What happened to the USA that they began to underestimate these folks?

The USA's extreme anticommunism led people to believe in its own supremacy and manifest destiny. Now, it is true that communism is a terrible system of economic organisation and capable of the casual deaths of millions, but that does not reflect on the intellectual capabilities of individual Russians. Or small teams that managed to achieve enough political protection to deliver great work.

It's something of an accident of history that HN celebrates Musk rather than Korolev, Tesla rather than Theremin.

Now it becomes clear where all that weaponry and defense systems used in Syria comes from

This isn't really a surprise; Russia has been arming Syria since its independence just after WW2. Much of the weaponry is the ubiquitous AK-47 / RPG-7 and clones; their air force uses MiG and Sukhoi aircraft.

Is it possible that the Russians have their own secret private sector building stuff?

No. They do have rather a lot of freebooter ""cyberwarfare"" people, though, usually engaged in credit card fraud and the like.