Hutchimus Prime
Adventurer
What I'd do depends on the situation. And common one cmes under the heading of In Death Ground, Fight. If I was an orc archer presented with the above situation, sure I would run. If I thought I could disengage safely the way the gnoll used a disengage move to prevent the opportunity attack at the cost of a little speed later on I'd do so. If I thought there was a chance of it working I'd call for Parlay. But taking opportunity attacks is the single most stupid and least self-preserving thing the orcs could possibly do. And gave the PCs three opportunity attacks. Withdraw and scatter (one in each direction) would have been a sensible approach. And would have had the benefit of making the PCs decide whether to split the party.
Mearls was not playing the monsters as if they were living, thinking beings suffering from fear. If you are scared of someone you do not drop your guard against them. He was playing them as if they were game pieces in a scenario that wasn't fun any more so they took the single action that would get them killed the fastest so they could move on to the next scenario.
You seem to be applying the tactical rules to the scenario rather than role-playing. Two flavors, both equally valid.