Based on it's shape, it seems the business end of the tube is where the majority of the pressure difference would be created. I suspect that the mask is more to prevent air rushing into the nose/mouth than providing the seal for the suction. With the curved tube extending into the back of the throat, and at the size it seems, I suspect that the soft palate and back of the throat would provide a sufficient seal, with enough pressure applied to the mask and a sharp enough pull on the plunger.

Also, many sailors on industrial boats that I've seen have beards, so I'd suspect that it was a consideration in it's design and probably why the tube is there, instead of just a mask, like the yellow plunger thing.