Those are all reasons for not manufacturing fuel in orbit, but the person I was replying to was talking about processing materials from other planets. I get the desire of Mars-as-a-gas-station/space-oil-field, but don't see a benefit to manufacturing other things there specifically because of gravity. If we have the ability to ship materials from planet to planet for processing, then I don't see how you get much benefit flying them down to Mars's surface for processing versus just staying in orbit and shipping fuel/whatever you get from Mars up to you, just like with the other planets.

Not planets, asteroids. Will Belters be able to refine and create high tech alloys in zero g? Will they be able to manufacture parts and equipment in zero g, at scale. It may be that all those activities require gravity, or are exceedingly difficult in zero g, we don't know. In the case that those things are hard in zero g, my proposition is that Mars will be desirable place for those activities due to its low gravity and atm making it easy to send back up.