The same applies both in the vacuum of space and the atmosphere of Earth. If you throw a ball at a spaceship, the spaceship's measurement of the ball's speed will vary depending on the velocity (velocity includes direction) of the ship. For example, if the ball is thrown at 40mph, and the ship is flying towards the ball at 40mph, then the ship will measure the ball's speed as 80mph. If the ship is flying
away from the ball at 30mph, then it will measure the ball's speed as 10mph.
Whereas if you shoot a laser at a train, the train will see the beam moving at the same speed regardless of the train's velocity.
Sylas set the second scene in a vacuum because air on Earth will slow the speed of light to slightly less than 299,792,458km/s. The difference is hardly anything, and it's irrelevant to what we're talking about anyway.
Please correct me if I'm wrong, I'm here to learn.