The crazy-hard deadliness of the Irontooth enouncter has to be intentional. It's 1evel 6, for a group of 1evel 1 PCs, which by the book is just on the edge of survivability, if they spend all their dailies and action points. And one of the monsters is a level 3 elite, which, again, pushes him into the edge of survivability range.
And it's an encounter that the adventure kind of funnels the group into: It's the only way for the players to figure out that there are bad cult happenings at the Keep unless they have the "heroes on a quest" kind of hook. Presumably, if they're heroic types, they'll be willing to make a quick (and paid) detour to help the good citizens of Winterhaven out. And even if they're not initially interested in the kobolds, the kobolds attack them, repeatedly, and are incredibly annoying. My group decided to wipe them out on principal.
My theory on this is that it's the same reason there's a dragon, a solo monster several levels above the party, at the end of Kobold Hall, particularly the demo version. The word on the street (by which I mean the internet) was that it's impossible to kill low level characters in 4e. By putting a very hard encounter at a key point in the flagship adventure, that everyone who's interested in 4e will hear about if not play, the designers send a very clear signal that this is not true.