And notably, the "drones" were civilian propeller aircraft fitted out to fly an unmanned suicide trajectory. I'm not sure they would even look all that distinguishable on a SAM operator's screen from a small jet like this.
The Airliner has a transponder and a radio. Pretty sure the drone does not.
The transponder code, assigned by various ATC would identify that aircraft as a civilian airliner when it checks in, and on the screens of the SAM operators.
Also, the speed and altitude of the airliner, even approaching Grozny would not be the same as a drone. Airliners, even on approach, are somewhat faster, probably 200-250mph, or faster, and much higher in altitude, at least 5000ft, probably more like 10,000ft until close to the airport.
Out of curiosity, why wouldn't a hostile power also put a transponder on their drone (maybe one even replaying a nearby plane's code)? Surely that could help it blend in and avoid defenses
More importantly, it's not uncommon when crossing Air Route Traffic Control Centers (ARTCC) regions (eg. from Washington Center, to NY Center) for controllers to instruct pilots to change Squawk codes. Same applies when crossing from one country's airspace to another.
One of these drones, without a bunch of extra avionics would be unable to change transponder codes in flight, and talk with controllers via relay, that would probably double the cost of the drone, or at least significantly increase it.
So even doing something creative, like spoofing the transponder Squawk code, from another aircraft, probably wouldn't help.
Also, with Mode-C, and Mode-S transponders, the later used with ADS-B, which feeds all the flight tracking websites, the transponder transmits altitude.
A SAM operator will figure out somewhat quickly if an airliner is supposed to be at 10,000ft and 250mph but isn't according to primary radar tracking, but much lower and slower, that it's spoofing it's transponder.
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.
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).
It's one of the reason Russia was very hesitant to shoot them down initially. Some of the planes were cessna and similar single engine prop planes that were loaded with explosives and remote controller:
The transponder code, assigned by various ATC would identify that aircraft as a civilian airliner when it checks in, and on the screens of the SAM operators.
Also, the speed and altitude of the airliner, even approaching Grozny would not be the same as a drone. Airliners, even on approach, are somewhat faster, probably 200-250mph, or faster, and much higher in altitude, at least 5000ft, probably more like 10,000ft until close to the airport.