Rocket engines don’t just immediately start, propellant has to be flowing through the turbo pumps before the flames start. Same at shut down. On the video you can see vapour coming out. That’s either methane or oxygen. How much of either? We can’t say because spacex doesn’t talk much about it. Same with ‘venting’ which happens quite a lot as shown on the videos as well. Whether it’s a little or a lot, it’s definitely more than was there before launch.
And that’s on a successful launch. Scott Manly talks about the last launch and shows how the methane levels were draining on the ship way faster than the oxygen levels, pointing to at least incomplete combination and probably methane puking out the back. Methane may be ‘clean’ when it’s burnt under optimal conditions, when it’s conflagrated in a RUD it’s less so.
Yes, yes, “it’s a drop in the bucket, it’s a tiny percentage of blah blah blah” which works fine until they start launching 3 per day. Then the question of what methane (a highly potent greenhouse gas) does when it’s directly added into the upper atmosphere gets answered.