More precisely it was a “civilian” Apache operated by Boeing for the tests, modified from the US military production line.
It was flying with a US civil aviation “N” registration painted on the tail boom. Which is the white blurred out mark on the tail in most public images from the tests. And hadn’t had any “United States Army” markings applied.
Also somehow they remain yet to address the existence of MMW obscuring smoke grenades such as M81 or why such a feature couldn’t be implemented, It’s not like IR Obscurants aren’t either.
As proof that they do exist and entered service you have the US’s 66mm (IR/MMW) M81 obscurant smoke grenade;
Matching exact wavelengths isn’t constraint on any of the radars or RWRs (see older band resignation from US to NATO standards), even when specs are know for both transmitter and the receiver why would it be in this case.
Doesn’t help that the MMW band covers 30~300Ghz, so would cover both the 35 Ghz and 94 Ghz seekers of the Hellfire and Brimstone respectively.