I doubt any of the famous three letter US agencies would need (or want!) such a wide open door as a "specific URL" and logging in with an administrator account acquired from the firmware itself. I would think that they would want a door that couldn't be opened by someone who happened to spot it, to not give anyone else the same advantage.

No, this is more likely the result of a test or some annoying procedure during development that needed more access than the device would normally provide - I've seen it before, and I think most people who work with embedded systems at least at some point in their career have done something that would be bad if it ended up in production units.