In fairness to that guy, it’s a pretty natural assumption to make that something being sold as “an adventure for characters Xth to Yth level” would be able to be run out of the box without a ton of additional work on the DM’s part. New DMs look to these things specifically because they haven’t learned those adventure-building skills and they want something that will tell them how to do it step-by-step before jumping into the deep end of designing their own adventures.
Oh, absolutely. But they still need to read the thing. Nothing is more tedious than playing in a game with a DM who hasn't read the module.