A regional jet on approach and a prop aircraft in cruise don't necessarily look that different in ground speeds, altitudes, or even radar cross section to most radars.

The regional jet is sqwalking on ADS-B, though.

Yeah but the ADS-B transponder didn't know its own location because GPS was being jammed by the Russians to try to force the drones off course.

Why is the jet even flying in an active war zone?

This is the Russia war coming home to roost. They better admit that they are engaged in an actual war, and stop allowing civilian aircraft in areas that are attacked frequently.

Secondary surveillance radar does not depend on tracked aircraft knowing their own position, though.

ADS-B is an augmentation of that, which makes receivers simpler, but ATC generally does not rely on it exclusively (except in some very remote regions), nor on any type of active/cooperative signal or response – if everything else fails (maliciously or accidentally), there's usually primary radar as well.

These "drones" are more like enclosed ultralights, heavily loaded, 50-80mph, which an airliner would have already stalled at and be dropping out of the sky.

Ukraine is modifying a large variety of smaller aircraft to be suicide drones. Yes, A-22s/A-33s are used which cruise at like 100-120MPH or so (though there's been some talk of turboprop conversions of the same, too). But other small civilian aircraft which cruise at more like 160MPH have been employed, versus a late approach speed of the Embraer of 180-190MPH or so.

And remember, radars vary groundspeed, which can easily vary by +/- 25MPH from actual (and will be reading the Embraer's speed on the low side).