A rocket in vacuum doesn’t “push” off of anything. The directed expansion of gases causes the system to expand linearly in both directions. The rocket goes one direction and the gases o the opposite direction. In days of old when there was no concept of vacuum and it was supposed that space beyond the atmosphere was filled with a substance termed ether then it was believed that push was the operating force.
The rocket is pushing off of the exhaust gases.
It is a simple and imprecise but essentially accurate way to describe the law of conservation of linear momentum in third grade language used by most mass media.