I, too, saw this on aeroroutes.com.
I wonder what the purpose of this particular flight is as I, too, find it rather strange.
If LOT Polish Airlines announced a regular scheduled service to STN, that would be something else but to add a totally one-off scheduled flight as they have done seems rather strange to say the least 🤔
LOT Polish Airlines are no strangers to the airport having previously served STN in one way or another in the mid 2000s via their short lived low cost subsidiary Centralwings.