Hi Severac, thanks for the clarification! This is what the operator mailed back to say as well. I had completely missed the time difference.
However, I am still feeling that arriving in Belgrade late at night is not the best, so I would like to figure out what operator NIŠ EKSPRES is offering. The BAS timetable (searched based on weekday) has a result with start station "Prolom Banja" (7:30), but I googled the location and it does not make sense to me that the bus drives away from Belgrade to Sofia, and then north-west back to Belgrade. Is this result ignoring the location parameters I'v