From my reading of the article I believe the author isn't talking about the type of attacks you are making, but just that you should all(as a group) be focusing on the same targets.
This is the only time he really mentions area attacks vs single target attacks. That quote is absolutely correct and the point I think he is trying to make is to be careful about the powers you choose.
Yes, he is discussing focusing fire.
But the fact that he is not addressing multiple target attacks is telling.
Some characters—particularly controllers—can’t help but spread their damage out. That’s okay
He is missing an important aspect of focus fire in his discussion. He is missing area attacks. Every PC that can take area attacks in the game system, should take area attacks.
A group with nobody who takes area attacks will do LESS focus fire than a group that has many PCs with area attacks.
The reason is that of increased overall damage. His statement here is totally misleading:
Because most monsters fight at full potency until they reach 0 hit points, only the last point of damage you deal—the one that reduces a monster to 0 hit points—makes any significant contribution to winning the fight.
This is wrong. All damage makes significant contributions to winning the fight.
If one has a 50% chance to hit a foe, he is doing 50% damage. Attacking two foes instead of one averages 100% overall damage. Attacking three foes instead of one averages overall 150% damage. Even if the area attack does 80% of the damage of a stronger single foe attack, that's still 80% damage spread between two foes and 120% damage spread between three foes. Both of these are more damage than the 50% average damage of targeting a single foe.
And by definition, it is difficult to not attack a foe who is already damaged if a PC is throwing out area attacks most rounds. The PC is bound to sometimes or often attack an NPC that some other PC or PCs are focusing on.
Focus fire is important. No doubt. But, area attacks which he mostly avoids in his discussion and he actually cavalierly dismisses by dismissing the spread out damage of controllers, will actually end an encounter quicker than just using single foe Focus Fire attacks.
Let's take the example of a Striker and a Controller. The Striker averages 10 points of single target damage per round. The Controller averages 5 points of damage per round and can attack n-1 foes per round (if there are 5 foes, he can attack 4, if there are 4 foes, he can attack 3, etc.). They have 5 foes with 30 hit points each.
The Striker attacks one foe every time. The Controller attacks the foes that the Striker is NOT attacking.
Round 1: Foe A (10), Foe B (5), Foe C (5), Foe D (5), Foe E (5)
Round 2: Foe A (20), Foe B (10), Foe C (10), Foe D (10), Foe E (10)
Round 3: Foe A (dead), Foe B (15), Foe C (15), Foe D (15), Foe E (15)
Round 4: Foe B (25), Foe C (20), Foe D (20), Foe E (20)
Round 5: Foe B (dead), Foe C (25), Foe D (25), Foe E (25)
Round 6: Foe C (dead), Foe D (dead), Foe E (dead)
Here is an example of the Controller NOT doing focus fire and the foes get 23.5 attacks against the PC (assuming a dead foe gets to attack an average of 0.5 times in the round it was killed).
Let's compare it to two Strikers that ARE doing focus fire.
Round 1: Foe A (20), Foe B (0), Foe C (0), Foe D (0), Foe E (0)
Round 2: Foe A (dead), Foe B (10), Foe C (0), Foe D (0), Foe E (0)
Round 3: Foe B (dead), Foe C (0), Foe D (0), Foe E (0)
Round 4: Foe C (20), Foe D (0), Foe E (0)
Round 5: Foe C (dead), Foe D (10), Foe E (0)
Round 6: Foe D (dead), Foe E (0)
Round 7: Foe E (20)
Round 8: Foe E (dead)
Here, the foes get 21.5 attacks against the PCs. This is only a slight improvement.
The foes get 2 fewer rounds to attack in this second case, but the first was a scenario where the Controller does not attack the foe that a Striker attacks. If we change it so that area attacks are combined with focused fire, then it becomes:
Round 1: Foe A (15), Foe B (5), Foe C (5), Foe D (5), Foe E (0)
Round 2: Foe A (dead), Foe B (10), Foe C (10), Foe D (10), Foe E (0)
Round 3: Foe B (25), Foe C (15), Foe D (15), Foe E (0)
Round 4: Foe B (dead), Foe C (dead), Foe D (20), Foe E (0)
Round 5: Foe D (dead), Foe E (5)
Round 6: Foe E (20)
Round 7: Foe E (dead)
It drops to 19.5 rounds of attacks by the foes. This is 4 better than the first case of 23.5 rounds.
Area attacks are a significant contributing factor to focus fire and the author mostly ignored/dismissed them.