1. HN is not "an informed audience", it's literally the public. And it's common for software developers to mistake themselves for domain experts on any domain but they aren't.
2. It's not an informed audience if misinformation is not challenged, it's a misinformed audience.
3. The flu is a bad parallel. The flu, while annually killing lots of people, does not overwhelm hospitals like a novel virus does. At best it makes people think this is like swine flu, while it has quickly shown to be nothing like swine flu.
Regarding 3, upper estimates for the 2009 swine flu are north of 500k deaths and the 1918 Spanish flu is 50 million.
Flus can be very deadly and the poster obviously did not intend to use the word in a diminutive way. You added that interpretation, knowing it is wrong, and then challenged it.
The whole post was playing down the effects of the current pandemic on society. The whole conclusion was that "a flu won't cause [society to break down]".
So yes, the poster was using flu in a diminutive sense. And yes, flu is deadly and a lot worse than a lot of people (who've probably never had flu) realise. But even given that, even recent novel flu strains haven't stressed and shut down the global economy like this pandemic has.
This is juxtaposition, a form of literary style. He is sarcastically downplaying the _flu_ in comparison to the world wars because the _flu_ is already significantly lesser than the holocaust or an atomic bomb landing in Japan.