I think what some of my fellow flat_Earthers fail to understand is that once a molecule of rocket exhaust leaves the rocket, it can no longer have any further effect on the rocket. It does not matter if it hits the ground, or a wall, or the gasses of the atmosphere. It cannot have any further effect on the rocket because it never touches the rocket again. This is why it does not matter if the rocket is in the vacuum of space.
They are making an analogy to pushing off against a solid object. But this is a false analogy because the exhaust gas does not propel the rocket by pushing on the atmosphere after it has left the rocket. The rocket propels itself by pushing on the exhaust gasses and then allowing that gas to depart out the back end.
But none of this tells us anything about the shape of the Earth. The claim that space does not exist, and the claim that rockets cannot function in a vacuum, are entirely separate claims from the flatness of the Earth. There is nothing about a flat Earth that precludes the existence of space and there is nothing about a flat Earth that precludes rockets from functioning in a vacuum.
And when Neil Armstrong travelled to the moon and back, the Earth being flat did not prevent him from getting there, screwing up his big line*, and getting back.
* He was supposed to say "One small step for a man, one giant leap for mankind." But he left out the indefinite article, making the line nonsense, since "man" by itself, without an article, is synonymous with "mankind." I'll bet he never went to bed at night without feeling embarrassed over screwing that one up. Still, he was a pretty cool dude.