There was no supervisor from what I read. There was someone doing COORD and someone training on ADC with an OJTI watching. Perfectly legitimate. The COORD position sounds like they had OCA at the time but they were not a supervisor.
I am not saying in that instance that the aircraft were assigned their own separation, it was just an example.
Having read the interim information and knowing the location of the tower it seems perfectly plausible to me that the tower could visual separate the aircraft at night with the go around off 34 passing behind the 27 go around. Without more timings or an actual radar paint that is speculation on my part but the OJTI and ATSB also thought it was not an issue.