Its very easy to effectively get softlocked on muluna since you have no way of producing atmosphere on planet when you first arrive, you need to send your ship back to nauvis and pick some up, and you can't produce any until you get to muluna so its not possible to come prepared with it. This would normally not be a problem, but you can't produce thruster fuel or thruster oxide in space until after completing most of muluna.

This is the case with many builds, usually associated with mods to complicate the gameplay.

Also, why is Muluna 12,000 km away from Nauvis?

This is how the author of the build configured it.