BBQ, Smoking, Grilling, it is all so subjective based on who you talk to in different parts of the country its not or shouldn't even be an issue. Generally speaking IMO anyone that uses "purist" in the same sentence with bbq, smoking, or grilling already has missed the boat and knows not what he/she spews forth.
You want to get down to brass tacks anyone that uses any type of grill, smoker, or otherwise man made material, fire containing apparatus. is not a "purist".
Its widely accepted at least in this country that original true "BBQ" as we know it was nothing more then a pit in the ground, assorted burnt down to coals wood, and the animal thrown in on top and covered.
Brought here by the slaves by way of the West Indies.
I say...BBQ should be anything that associates food cooked in a manner that inspires communal gatherings around a heat source, fostering happiness and claims that what is being consumed is the best there is.