I think one possibility as to why only 1 aircraft acted on the RA is the fact that the controller picked up their mistake pretty quickly and cancelled the climb/started the descent.. Not entirely sure how pilots are trained (yes I'm the other side of the radar), but I suspect if you had just been issued climb, then as you were leaving the level you got told "safety alert, cancel climb, maintain FL380, traffic 3 miles 12 o'clock 800ft above" and then you got a RA telling you to descend you might not feel the need to tell the controller that TCAS was telling you to do what they had just told you to do..?
Anyway just a thought.
Also seems to me that the media seems to be glossing over the fact that the controller realised the mistake and acted to fix it immediately. Not sure if it was before or after a STCA. Still a huge mistake, but it would be nice for some balanced reporting.. But I guess "controller makes mistake then rectifies it, and along with the added safety net, crises averted" doesn't strike the fear into the flying public as much as making out that an (overworked, undertrained, reckless) air traffic controller pointed 2 at each other and if not for TCAS/quick acting pilots we would of lost 600 lives.
May be wrong, but I'll make my mind up when some more facts are made available. For all we know it was an experienced, well trained, well respected controller who made their first major error in a lengthy career..