No, I think what he means is that solos may end up being much easier to defeat than what their xp budget might suggest. However you look at it, the party will on average, out-action it 5-to-1, and this is not taking into account spells which inflict status effects, such as aforementioned certain justice. Simply put, it is all too easy to just focus fire and debuff a solo to near uselessness.
Well, perhaps, to an extent - but it depends on your party composition. Paladin's simply are better on 1-1, so a party with a fighter will be less overpowering.
For example, I think I would sooner face a solo white dragon than 6 needlefang drake swarms.
Which says more about the needlefang's than the white dragon, IMHO... If you look at the DMG guidelines there's no way the non-solo 1 level lower drake swarm should be able to have comparable defenses. And when it comes to AC and to-hit, soldiers (like the drake swarm) just are really good. The comparison is further skewed since the dragon's are brutes; their advantage lies partly in their higher hit points which aren't reflected in WotC's (flawed) hit point calculation method for solo's. A party with non-AC attacks might take the swarm on more easily, esp. if it's an area effect, and esp if the defense difference were there (as it should be).
Compare this with 5 normal foes. Certain justice is clearly weaker in the later case. The party will find it much easier to lock down a single foe (via chaining stun-effects or whatever) than trying to replicate the same stunt on 5 individual foes. That is the reason why the solo BBEG vs party scenario in 3e just did not work. And it apparently faces similar problems in 4e. Wasn't there a test scenario some time back where a lv30 party defeated the tarrasque without it ever getting a turn?
I just DM'd a 3.5 combat vs. an (effectively) solo BBEG, and the combat took several rounds of back and forth. I expect in 4e too there are strategies to minimize the impact of many status effects. Dazed, for instance, can be mitigated with action points, and specifically certain justice can be removed simply by staying out of reach of the paladin for one round - something a flying dragon, say, with 2 actions points should be able to achieve without great difficulty.
In any case, a certain justice does actually need to
hit to have effect, and solo's also have a defense bonus, so this is harder to do to a solo than to a normal creature.
So, sure, using reasonable tactics works well - but it's not a sure win, and it's an intrinsic disadvantage of single targets, which isn't necessarily a problem with the rule-set but a reflection of reality, insofar that applies here ;-).