I've just looked online at a map of where the A-50 was when it got hit, and it's well within range for am S-200 or S-300 fired from inside Ukrainian held territory.
The impact location was ~130KM from the frontline, an S-200 can kill a target of that size from over 300KM away (it's not a gaurenteed kill as it's on the limits of fuel but it can and has been done) so I would assume an S-300 can too.
Given that Ukraine has a stockpile of S-200 missiles they aren't using (they've been converting them to surface to surface missiles) they may have just said "Well the only thing flying over the sea will be Russian, lets fling a couple of these 60 year old boys that direction and see if they find anything".
Very dangerous but those S-200 missiles have been known to fly 200 miles off course and take out an airliner before... so indiscriminately throwing them at an area where Russia might be operating air traffic has a good chance of hitting at extreme range.
For example https://en.wikipedia.org/wiki/Siberia_Airlines_Flight_1812 (not the only incident).
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