JD-EE (I've even entertained the thought that the WX radar had somehow malfunctioned. But that would have to take place in such a way that the equipment itself would not notice and report it to ACARS for replacement at the next stop. There aren't many places that kind of failure can take place if even rudimentary monitoring is included in the design. It's another low probability thing. But it would explain their flying peacefully into disaster.)
It's a single string signal from the back of the WXR R/T down the waveguide, out the antenna, through the radome, to the target, and back again. The only monitor for that is the Mk-1 eyeball looking for ground or sea return with tilt at appropriate angle.
Good scanning technique is to adjust tilt so ground/sea return fills the outer half of the display, and any target that grows out of the grass is a storm.
There have been reported cases of heavy icing on the radome causing the radar display to go all red, which won't trigger a fault.
In this case, we just don't know if a pilot was using the Wx radar at all.
GB