Transcripts from the control tower are now available, which confirm that the small aircraft did not have permission to be on the runway, and the JAL Airbus did have permission to land. Hopefully there’s a surviving flight data recorder from the smaller plane which might give some clue as to why they thought they had clearance to go onto the runway. The pilot of that aircraft did survive, so in time he may be able to give an account of himself.
https://www.bbc.co.uk/news/world-asia-67870119