@Dr. Jan Itor bhai has answered it spot on.
Firstly, the problem is DNE( Do-not-exercise ) was removed by circular in OCT-2021 but wasn't known mainstream. Even I didnt know usually i tend to "stay updated".
They say DNE was for STT trap, since there is no STT trap, no DNE. But DNE is actually the fundamental logic of an Options Contract. Buyer is not obliged.
Secondly, taking counter set-off position is possible but as quoted in above post, the buy side is usually blocked etc. Only way is to short Futures or short some counter position.
Now shorting means huge margin requirements, especially in the range of 50% during Expiry week.
So the buyer, hence termed small retailer, bought a few "gambling" lots is in no position to bring it especially in the last hour.
The whole synchronized or orchestrated method of ("sellers" in this case) one side disappearing will have future consequences too.
If the exit is in sync, then the same group could've hoarded the underlying too bcos all these guys would go to auction in short delivery.