The standard encounter building rules suck and the CR system is all over the place.
I ran the adventure and encounter myself, for a couple of newbie, never played an RPG before, 13-year-olds choosing the party's approach. They didn't have any trouble with it.
So, by all means, continue to think the encounter is unreasonable if you wish. Your prerogative. But it doesn't match my experience. Fact is that LMoP has a reputation of being an outstanding module for good reason - and that reason is not over-powered encounters.