You aren't completely wrong as Newton's third law applies, but the forces you speak about are opposing in a different direction than a rocket would be on a shuttle. If an astronaut pushed on a shuttle, Newton's third law would state the shuttle pushes back onto the astronaut. I agree with that. But now imagine the astronaut as the opposing force of space (minimal), and now apply a force to the shuttle in the direction of the astronaut, as a rocket would. There isn't much in the way of the shuttle and Newton's second law is then applicable.