> how then are we explaining the appearance of these things on several different sensor platforms
That would be trickier to explain, but we should be careful with the story telling here, they report instances of many visual anomalies, and slotted in only one story where there was a ground radar anomaly preceding the visual one that did not match exactly, i.e correlation with very low sample size is pretty meaningless.
...Knowing that, it's still interesting to seriously consider it anyway - visual systems use light, and radar uses radio, both are propagating electromagnetic waves of different frequency, they permeate solids (or not) differently, but they are all share similar basic behaviours i.e both are sensitive to density of the medium and can experience refraction and total internal reflection. So it's not unthinkable that a single event could explain both.
> Presumably the people who design these radar systems try and make them account for air density etc
I'm sure they do, but they cannot accommodate phenomena that they are not yet aware of. And even if we figured this out it's probably going to be difficult to tune those anomalies out in an automated way, in the same way that it takes a human to understand that a mirror is a mirror by analysing the behaviour of an image... perhaps with multiple radar from very different origins that do not concord they could be filtered out in a simpler way.
[EDIT]
Interesting section in the wiki on radar [0]:
> Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground, sea, and when not being tasked for meteorological purposes, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections, meteor trails, and Hail spike. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff [1].
Perhaps a more likely explanation is perceived correlation between a short freak weather even in an area with lots of radar focused on it, e.g a super dense and short hail storm that suddenly drops like a stone. followed by these more common visual phenomena.
[0] https://en.wikipedia.org/wiki/Radar#Clutter
[1] https://en.wikipedia.org/wiki/Clutter_(radar)