It must be quiet in PO. I just tested it and it wouldnt let me battle because it had timid. They said that to use eruption on heatran it must be quiet. Go change it in the teambuilder.
So through logic I can infer that the event must be quiet since in PO they only accept Pokemon that are obtainable with their possible movesets ingame.
Additionally this website shows that the event heatran with eruption must have a quiet nature.