It also appears to have a fairly narrow detection angle. This might work for spotting a drone when you already know roughly where it is, but that problem becomes infinitely harder when you have to scan the entire sky.

RF drone detection has been a challenging problem for quite a while. Lots of solid state radar/RF detection products have emerged in the space, but it is not a trivial problem. And that is for drones with active RF comms, anything flying autonomously is even harder to detect at a far enough range to actually do something about.

Is that a limitation of the antenna? I though QuadRF uses SDR so can see many frequencies, not just the wifi things like ESPARGOS [0]

From documentation, QuadRF: Operating frequency range of 4.9 - 6.0 GHz (C-Band).

0. https://espargos.net/

for lack of directonality?