I dont think its quite hard at all for the Edbens to remain a secret to the US at large and other powers. If we grant that their renaissance era is 1:1 to Earth... then most of that planet isn't traveled by most of its natives or mapped by its natives and there large patches of planet that are still hunters and gathers.
On Earth during the renaissance era, there would have been tons of places to place a colony that would be isolated from the natives, to the point where it would take them days or week to reach but mere minuets for the Americans to transverse with aircraft. And this means if the colonist explored around their city, on foot or motorized, they'd be unlikely to reach a village as well.
As for everyone else in orbit, the civilization on the planet would be very hard to tell they exist. First of all, if they happen to just look down at the planet, most of the planet would be empty of native life, so its unlikely that they can just randomly find them. Doubly so, if the station in orbit, was over a region that was devoid of life. The other two big things that be easier to tell from orbit aren't happening on the planet yet. They aren't emitting any EM radiation. And when the planet is dark, their urban centers aren't lit up.
Now if some third party group, were to do a detail examination of the planet, without landing on it, they could tell that renaissance era cities were a thing, with extra amount of sulfur and other burning fuel waste would leave in the atmosphere.