Most major airports will already have a counter-UAS system, it's a huge industry.

One big issue with radar is that it has the same problem pilots and human observers do: it struggles to distinguish drones from anything else in the sky (birds, balloons, planes, etc.). This is an active and improving research space, but by and large with radar, when your pilots report a drone, you still don't know how to figure out if it's the typical mis-identification or something real.

If would likely need to track them well (not sure from this article/video if that's the case?) to be useful in that scenario...

Drawing a splodge in roughly the location (not sure if there's range info either? I doubt it if it's passive) overlaid on the video likely won't cut it...

Yes, primary radar has been useful for detecting airspace incursions since 1939. Nothing new here.

Phased array antennas (in use since the 1960s) and AESA (in use since the 1990s) are very mature tech that RF engineers are well aware of.

This gizmo is primarily interesting that it's pre-packaged at a price that hobbyists can afford.

Only the ones that use radio for control. The fiberoptic ones are "dark" to this setup.

There are more way advanced systems for cuas, where they infuse radar and visual and acoustic plus now AI to minimize the false positives, but practically speaking, they are not bullet proof and still fail. RID (remote ID) is a way to have a cooperative communication and was mandated in US, but there are ways too to spoof it and cloak it.