The Blueprint item from the expansion Widow's Walk states that you can move between rooms with dumbwaiters. Does this mean you can move between any room with a dumbwaiter even on a different floor? If so that makes several haunts unplayable if the Traitor has it.
You might be missing the dumbwaiter icons in general allow anyone to move to different floors; that is their primary purpose.
Some room tiles include an elevator-like symbol on them, in addition to any Item, Event, or Omen symbol they might have. This symbol signifies that the room has access to a dumbwaiter, which players may use to move to a Landing one floor up or down. It costs 1 space of movement to use a dumbwaiter, plus 1 space to move onto the Landing.
So while it costs an extra movement, you do not need the blueprint item to make use of the dumbwaiters.
What the blueprint allows is to move from one dumbwaiter icon to another as a movement, as opposed to the normal use which requires you to move to the landing of a floor 1 below or 1 above where you are. So yes, the blueprint makes the dumbwaiters better, but it does not give them the ability to move to a different floor; that already was part of the dumbwaiter rules.
Yes, the lack of restriction on traveling to other floors is intentional. After all, that's the whole point of dumbwaiters. They are miniature elevators meant for food or other goods.