Kind of like saying 10 plants per pound?
Sorry that was all in fun.
Seriously, are they growing in party cups?
The yield per plant is highly variable depending on the grow type, strain, light quality, veg time etc. It's not uncommon in the indoor grow industry to get 4 ounces from one plant. More than that and the plant takes up as much room as two plants that have been vegged less, so the difference is really moot. 4 ounces is a good upper limit approximation. That would take 4 plants per pound. 10 plants per pound is more like the lower, conservative limit. Given that beginners are going to lose some of their crop, shit happens, it's actually a better number to start with and darn convenient for the sake of calculations. But if you just double the figure that puts you close to optimum conditions, and even then the number he gave can't be right.
Edit: please note that in the numbers above I am talking about yield of high quality flowers, not total mass of the plant with all the leaves. Also, the plants they show hanging in the pictures are less than 4 oz each. Their pot size also does not support huge yields per plant: