I think the statblock for the Barlgura changed between Thunderspire Labyrinth being conceived and typeset.
There's a couple of reasons for that belief; one is that the writers don't seem to know how 4th Edition Barlguras work, mechanically. The other is that they put a lone Barlgura up against a five-man party.
Barlguras, for those not aware, are a type of brutish low-level demon. They're a natural fit to a location called the "Well of Demons" and indeed they feature throughout the area.
The Well of Demons utilises a system of random encounters, much like the ones I've described earlier. Rather than providing interesting sidequests, these encounters are designed to discourage adventurers from slowing down while assaulting the Well. The Well works best when it's blown through in only two harrowing adventuring days, and the random encounters are a rather clumsy way of expressing that.
It works like this: you roll on the table each time the adventurers take a rest, whether short or extended, and apply the resulting events. Consequences range from the spooky (ghostly minotaurs haunt the players) through to the disturbing (a rift opens to the Abyss), with a dash of the oddly helpful (a quasit shows up and offers to answer the party's questions in exchange for cash).
One of the encounters is a lone Barlgura.
The Barlgura is a level 8 brute. It's got a lot of HP and it hits hard, but it's got a very narrow range of attack options, isn't very mobile, and has defences that would look poor on a monster two levels lower.
A fight against a lone Barlgura goes like this: the defender tanks the Barlgura, and then everyone else drops their highest damage encounter power into it. It lasts a maximum of two rounds and doesn't move or do anything interesting. It gets two attacks a round, each of which is at a less than 50% chance of hitting a level 6 defender's AC, for a maximum of 62 damage if all four crit, or a more likely output of about 20 damage if two hits land and do average damage.
The average damage over its lifespan is only slightly higher than a healing surge, and that's assuming it doesn't roll badly and the PCs don't drop dailies.
In short, the Barlgura doesn't reach the minimum threat level - it's simply not capable of costing the PCs any meaningful resources that won't renew at the end of the enccounter. Rather than actually running the combat, the DM is better off just asking players to describe how they think the fight is going to go, and letting them off unscathed providing they describe an awesome enough battle.
Minimum threat is something 4E struggles with. The line between a weak encounter and a pointless one is very fine. Smart, optimised parties can find same-level battles so trivial as to be dull. Newer groups or ones with deliberately gimped builds will often struggle with those same encounters. It causes problems for module writers - how do you write an encounter that challenges a good party without making it a TPK for a more casual group?
It's not a question that Thunderspire has good answers to.