When you build them on earth, you aren't using "a factory", you're using a supply chain with thousands of factories and millions of people. A factory that can go from regolith all the way to spacecraft is still very speculative.
As for 2 billion tons of rocket fuel, it's not trivial but it's something less than 5% of one year's current CO2 emissions, and we're talking about spreading it out over a couple decades.
As for 2 billion tons of rocket fuel, it's not trivial but it's something less than 5% of one year's current CO2 emissions, and we're talking about spreading it out over a couple decades.