Yes. this is essentially accurate.
The sinuses are going to absorb substances through osmosis, eventually through the tissues and into the bloodstream. This process essentially involves a substance being made into a solution of sorts by coming into contact with the mucous present in the nasal passages. Once this "solution" is created, it can then be absorbed through osmosis in this fashion.
In short, you need a moist environment for the osmosis to work. Any dry material will be inhaled into the lungs, at which time, a similar process will take place in which the alveoli of the lungs absorb the material and introduce it into the bloodstream in a similar fashion. In all likelihood, given the environment of the sinuses, throat etc. very little actual powder should reach the lungs proper. Most of it will end up stuck to the walls of the trachea or throat on its way to the lungs. It just so happens that the sinuses are especially absorbent. Essentially any tissue of the human body is capable of the same exchange in a similar way. For practical reasons, insufflation just makes sense.
You certainly don't need to worry about "losing" your drugs though. Once it goes up your nose, it's going to be absorbed by the body in one way or another.