Am going to give you an explanation that you will not like because it is technically sound. I expect you to dispute it. I do not post this to change your mind but this is for the benefit of the silent readers out there who want more than just what Iran says about this event.
Look at this F-16 inside an EM anechoic chamber...
Why did we bothered to put a 'non-stealth' fighter into an EM anechoic chamber?
Most people thinks that an EM anechoic chamber is
ONLY to measure the 'stealthiness' of the F-22 and similar. That is not true. An EM anechoic chamber is usually used to test the radiation pattern of various transmitters on aircrafts and even cars. BMW tested their cars at Benefield Anechoic Facility (BAF) at Edwards AFB, US.
For receivers, while an aircraft is inside the chamber, its antennas can be tested and measured to see how many directions can the antennas receive and how efficient are those receptions. In the reception process, efficiency can be a negative because that efficiency can produce interference from many signals when the intention is to focus on a narrow range of signal(s).
So when we put a 'non-stealth' fighter like an F-16 into EM isolation, we are testing the various communication methods like common HF, GPS, Link-16, and weapons guidance that can come from ground sources. We want to see which signal can interfere with what, from which direction(s), and when. In combat, these communication signals are active at various times and even at all times.
The installation of RAM on the RQ-170 is not to reduce RCS, although that is a benefit. The drone is unmanned and remotely piloted. It can also receive various communication signals from other sources. The absorber is there to primarily control interference. If there is a reduction in RCS, great. If not, we do not care. The remote pilot feature is satellite relayed so that mean the antennas for that are topside. Absorber reduce and even eliminate any interference from edge diffraction signals that came from radar or any common EM transmissions that can negatively affect those topside antennas. The transmission/reception signals for the remote pilot feature should be as absolute integrity as possible.
You will not like my explanation despite being technically and logically sound. I cannot go any more details because that would enter the INFOSEC realm. You will not like this explanation because it reduces the propaganda impact this event. It is better and important for Iranians to believe that Iran was able to detect anything 'stealth' from America.