Import duties? Who cares about import duties? He's arguing that we need to leave the EU because their daft rules for how vacuum cleaners can be sold. This will make him free to make and market his vacuums however he wishes in the UK and outside the EU, but if he wants to sell them in the EU, he still has to play the EU testing and regulation games.
Except this time, he won't have any say in what those rules will be. Doesn't seem that smart to me.
OTOH, most of Dyson's income comes from outside the UK, most of his costs (apart from his offshored manufacturing) are in pounds, so a big fall in sterling makes his businesses much healthier. cui bono