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.
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.
We had several options for how to respond to this, and chose the most invasive, drastic and potentially economically catastrophic one.
In the absence of data around who is sick, who is not, and who is immune, there is no way to slow the spread of the virus during an outbreak except to keep everyone away from everyone else.
This does not minimize the harm a flu can do, it puts it in perspective. Nuclear bombs going off over LA, London, New York, and Beijing probably would not cause society to break down. That does not mean that does not mean that it wouldn't be terrible, deadly, and disruptive.
When planning for how to prepare and deal with this pandemic, people should not be preparing for some Mad-Max scenario where an assault rifle is their most valuable possession.