Possible cause of the Permian extinction

Researchers have found a 300 mile (that's around 480km in real money) crater beneath the Antarctic ice sheet that dates to around the time of the Permian extinction. Bolides seem to be at or near most of the major extinctions. I wonder, idly*, if the impacts themselves aren't the killer blow but rather the subsequent tectonic vulcanism.

* Idly = "wild eyed guess with no evidence or real understanding. Just for future reference, OK?