I think his theory is that smoke contains CO2 and would
help with plant growth which is not true as previous post
have explained the reasons in detail.
But if you want to add CO2 to a small grow room cheap
Set up a bottle that drips vinegar into baking soda.
A bottle hung upside down with a fish tank regulator
valve fixed into the top of the bottle to control the flow
of vinegar coming out, place a jar that holds baking soda
under the drip, a slow drip you don't want to have a mess.
As any one with basic chemical knowledge knows the vinegar
and baking soda reaction produces CO2.