I have no experience from these kind of cameras in real life. But in the article you reference it says:
"it is a good thing that the thermal imager can see through fog to a certain extent."
Sounds to me it is not totally unaffected. Also, the thickest fog in arma is quite thick and would deliver a massive amount of moist in the air. Seems reasonable to me it has a severe impact on the way heat can be read at a distance.
Found this folder from FLIR which backs this up as correct behaviour:
"Humid air acts as a “shield” for infrared