2.5 yes, under perfect conditions, and again I'm sure it wasn't his average, just his best ever.
If you have a carbon filter making a vacuum in the room, you can't seal the room. A true sealed room is air cooled lights. Sealed outside air piped in, cooling the lights, than the same air pumped out. Completely sealed wit minimal personal entering the room. A/C in the room to keep temp down. Even the reservoir should be outside the room. Sealed window to look in. Only this way is CO2 worth it, otherwise your wasting it, unless you have the money to pissed away. With this sealed room in place, there's no need for a carbon filter because the smell never leaves the room. The whole reason not to seal the room, and have air inlets is to exchange the air (CO2) in the room. There's plenty of CO2 in the air, normally, to have a very big harvest. It wouldn't be worth the effort for your small grow... imo. But I'll sell you some CO2 if you want.