I guess you missed the part where he was a henchman? At any rate, yes, that was a rare encounter, but there's generally one of those (rare, deadly encounters) in every adventure module. This is a tangent, and not really germane to the hit point discussion.
I just thought I'd point out that earlier editions are a bit deadlier than you put forth.
I was referring to averages here. Which your point actually nicely dovetails with. In 1e, it's a rare monster that has this kind of damage output. In 3e, it's an average monster. By and large, any CR=PC level monster can kill that PC in one round. It's not likely that the monster will, but, it's certainly within that monster's capabilities.
Which generally isn't true in AD&D. I mean, compare trolls. In AD&D a troll does 25 points of damage (IIRC - might be a bit more, but, it's less than 30). In 3e, that same troll is capable of slightly of 50 points of damage. There isn't that much difference in the HP of a 5th level character between editions.
Now, 4e reined that in considerably by jacking up the HP's of PC's. We're generally back to the three hits capabilities. It takes about three full damage hits from a level par baddie to drop a 4e character. Which is about the same as an AD&D baddie.
Which, rolling this all the way back around to the original point, means that, despite the very different numbers, adventure design between editions doesn't change all that much.