This fellow hasn't any idea of the reason behind the decision to divert. The captain is in charge and that's the end of the story. TG always used to carry far too much fuel going Australia, as they didn't comprehend the Australian rules for minimum fuel required in Australian airspace. They now do, and it is sufficient to allow for a timely decision to be made, in the fairly rare event unforecast weather eventuates at the planned destination.
However.... looking at the actual weather on that day and time, the wind was from the SE and runway 16 has a Cat 111b ILS, allowing for landings with no cloud base and 75m visibility minima. Thai (albeit a long time ago) were never able to maintain certification of the automatic landing capability of the old 747's (no idea if that applies to the aircraft used on this sector), so if a CAT 111b capability existed at Melbourne at the time, vis was reported at or or above 75m (and equipment may not be fully serviceable at the airport of course), it is probably safe to assume the aircraft was not capable, for one reason or another, or the crew were not qualified for such an approach. I think the suggestion that the passenger concerned get his pilots licence and start flying commercial heavy jets, before demonstrating his ignorance, a fairly good one.