Scientists agree that gas when expelled into a vacuum does 0 work (i.e., there is no momentum...it freely expands).
Scientists also agree the equal and opposite of 0 is 0.
They also agree that rockets work in a vacuum. Unless you can find me a credible source saying the opposite.
So someone has made a mistake, it's either you or literal rocket scientists. I'm going to say it's you, but I await your source.
Momentum is simply mass moving: mass x velocity.
Does the gas have mass? Yes.
Does it have velocity? Clearly yes.
So the momentum > 0. Conservation of momentum means rockets work, just ask scientists.