It's British (probably Commonwealth) English for serious or important. See usage 6: https://en.oxforddictionaries.com/definition/heavy (unfortunately, no dates given on the quotes).
This lists the usage meaning "severe" as from 1849: http://historicalthesaurus.arts.gla.ac.uk/category/?id=15861...
It comes from the idea of a "heavy wipe-out", which can quite easily occur while surfing "waves of consequence". Generally good, hollow, barrelling waves contain more water volume and energy, when they break they look and feel "heavier" and while being better rides, have more potential to injure a surfer. The slang has indeed made it's way into everyday usages in NZ and Oz and Hawaii, I guess California also.
I believe the original poster is describing a serious or unpleasant situation, which seems to be the case.
English is a Germanic language, so perhaps in a sense this meaning has always been part of it. As you point out, even if it's not considered completely idiomatic most people would have no trouble interpreting the meaning.
You've inserted yourself in a conversation occurring a tremendous distance from the events, and it's not like the comment was even making light of the situation.
And it was a conversation about a disaster as it was happening in complete real time, one which resulted in a loss of life, displaced families, and hundreds of millions if not billions of dollars worth of damage. It wasn't a past event which can be trivialised, it was being trivialised as people were being pulled from the rubble of their homes and workplaces.
This particular conversation was occurring between people, some of whom were far away from the events and some of whom were being shaken as they typed.
I also have prior experience with disasters, I provided technical support to the engineers in the NZ Urban Search and Rescue teams in Christchurch, and saw the devastation first hand as I was setting up remote offices within the red-zone and rescuing computer equipment from buildings that were missing walls. I also saw people working frantically to extract survivors from collapsed buildings as we went by. I lived in the same house and often shared rooms with those engineers that were doing the rescuing work, and I saw the harried expressions on their faces every day.
So I reserve the right to ask people to respect the situation at hand.