This is simply not true. I play a sorc hybrid build using affliction and crushing darkness. I kept getting the fearful debuff at 30+ meters. We started thinking it has something to do with dots. So when the right mob gets to 92%, 72%, 52%, etc I stopped dotting. Didn't get the debuff after that at 28+ meters. We we're also thinking that it may not be the distance from the boss you are dps'ing, but maybe where the other one lands. Either way, I am 100% sure you can still be hit with fearful at 25+ meters, just not 100% sure why. I completely stopped dps'ing and stepped back to test it, a couple of times.
I am a DF smuggler and I continually DPS both adds and haven't gotten the debuff or dot since my first attempt. You need to out range both of them. ie. be 25m or more away at ALL times. Stopping DPS might be fine for HM. but when nightmare rolls out you wont have the chance to NOT dps b/c you WILL hit enrage.