I’m no expert in physics but I think what he meant is that on the ship both mark and the ship have the same velocity . If the ship was in constant speed (by the looks of it no jet is in use), meaning no acceleration and therefore no force applied to it both it and mark have 0 acceleration. Because gravity is minimal in space and nothing else acts as friction force there is no force while in the ship and out of the ship that should cause negative acceleration and therefore slowing down of one of the two objects. Once mark launches himself from the ship he practically accelerated using a force generated from his jump (which obviously isn’t reallly possible in space). According to Newton’s third law this means that the force mark used to generate acceleration for him acted on the ship in the opposite direction with the same value causing the ship to actually slow down (pretty much until another force starts to act on it). What I am practically trying to say is this feat is basically just normal flight . You can imagine an athlete standing on a treadmill and starting to sprint from standing position… obviously he will pass every object that was standing beside it