This was brought up in the presentation as well - they have plans to eventually use the same technology they will use on Mars, on Earth as well, to take CO2 out

This was brought up in the presentation as well - they have plans to eventually use the same technology they will use on Mars, on Earth as well, to take CO2 out of the air and combine it with water to create methane and oxygen. If it happens, the rockets would be carbon-neutral.