That's a propane heater. It makes the greenhouse warm in cold weather to keep the plants alive and able to grow during the winter.

You wouldn't need a C02 "generator" because you can easily and cost effectively get CO2 in gas bottles to boost carbon dioxide in the greenhouse. It would be more expensive to generate CO2 via a chemical reaction and there would be no real benefit over just simply pumping in the gas from a tank. But yes, adding more CO2 to a greenhouse is a very beneficial thing which will promote plant growth and improve yields provided they are properly ventilated. There is just no reason to "generate" it in such an application.

https://www.greenhousemag.com/article/unvented-heaters-spell-trouble/

Is that basically just a propane heater? Personally I've always been a little sketched out by those for heating the shop because of CO.