If they weren’t technically in the military, would anything like a “stand down order” even apply? If they were private citizens, they did not answer to the US government. ?? (seems to me, anyway)
True, they were private citizens in the service of their country
Most of these civilian contractors were former 'brothers-in-arms' with the area military units, having served in the same units that were posted nearby.
The Secretary of State has an obligation to protect her own employees (the Ambassador); the civilians contractors were apparently on their own, as we found out later.
There were military resources nearby, or rather land based stationed minutes away; air resources were only hours away, with a drone overhead.
Even Neuland stated that they were watching the event "in almost real time", apparently by the drone overhead. There may have even been another drone nearby, possibly armed.
Previous requests for increased security went unanswered; in fact, the local security force withdrew immediately prior to the attack. Ambassador Stevens knew something was up.
Fundamentally, you are correct, the contractors as civilians answered to the government only as far as their employment; security wise, they were on their own.
But, their 'brothers-in-arms' knew of their situation, and were prepared to leave to give aid, but in the military, you await orders from your military superiors.
And there were no military orders forthcoming.
This was a violation of the military philosophy of "leave no man behind"