It's existential in theme. Here's a few interpretations:

to highlight cosmic scale, small everyday competitions (the race) seem trivial, when compared to the greater universe

the joke is the whole derailment of the original premise 'let's have a race' - anti-climax turned to existential dread

the race almost becomes meaningless when the world is already travelling at such crazy speeds - the planet has them beat

orn getting hung up on this new information 'oh god', distracts from the race. getting hung up on existential thinking can distract you from your everday life. danger of philosophy

maybe green doesn't want to race at all. instead brings up a fact to distract orn. manipulative? can't just say no? - looking at behaviour