Yeah, YEAH! I remember 777s taking part in SEAD (Suppression of Enemy Air Defenses) missions alongside some F-15s from the 48th Fighter Wing a few years ago, flying real low n' stuff
There is no reason why a skilful aviator shouldn't be able to fly an airliner fairly close to the ground.
Here's a KC-135 flying very low. And I am not suggesting this 777 was doing this. But even at 1000ft, chances of radar detection are significantly reduced.