From an airline pilot, 'Oceanic on Datalink Comms, there’s only one option, and that’s to hit the emergency button. There’s no lesser option, such as a PAN call or a simple heads-up, that you use on voice comms.'
My understanding is once they established radio comms closer to Sydney then then downgraded it to a PAN. Perhaps those questioning are accepting the media advice that it was initially a MayDay call, as in over the radio?