By the time we get to 10th level or so, PCs are pretty darn strong. If this is supposed to be a harder adventure, I would fully expect the final battle to be Lord Soth on a death dragon with an army of zombies.
This immediately makes me think of Crawford saying the encounters in this adventure are harder than we've previously seen, so that justifies the bonus feats being given to players. A dragon capable of turning the dozen low level NPCs nearby into a dozen zombies that act on its turn? That's certainly one way to make an encounter harder.
Still, something held back maybe.
What if Lord Soth is there too? Suddenly the stakes are much higher.