A "swarm" of a dozen of these big beasts, as reported, should be pretty easy for modern radar systems to spot, no?
If they are low, slow, and mostly made out of plastic, then no, they would not be easy to pick out of ground clutter.
How are you going to use doppler radar to differentiate between a drone going 80 km/hr at 3 meters AGL, and a truck?
The attack was coordinated with a rocket attack, so your radar would be dealing with shrapnel, smoke, and debris which would add to ground clutter. If the attackers were smart, they would have loaded the rockets with some chaff.
TFS says they were "swarm-like", but TFA does not use the word "swarm" at all. They may have all come in together on a single vector, but only if the operators were morons. There is no way that a defender should expect that from a competent adversary.