You ought to take another look. The one million year long basaltic flow was punctuated by Rhyolitic explosive super volcano like eruptions. The traps as now preserved would have covered in a thick lava flow an area like modern Europe. Originally, the indications are that it covered an area four times that size. It makes the Deccan Traps look small. We are mining copper and nickel in huge ore deposits dating back to intrusions under the mat of basalt and dacite. Then, you have to consider the coal deposits that were set on fire. The indications I am aware of suggesting an asteroid are of one which may have triggered this initially. The delta O18 measurements of the time show ocean temperatures of 40C.
An asteroid by itself brings on a so called nuclear winter. The rhyolitic volcanos would have put up a lot of ash which would cool things only in the very short run (a couple of years to a decade) but if rain wasn’t able to scrub the atmosphere, a lot of nasty gas would be left in the air.
The P-T impact was probably on the ocean somewhere, as the oceanic carbon shifted from C13 toward C12, which is something that volcanic eruptions won’t do; either the C12 was in the impactor, or the impact released methane ices on the ocean floor.
These large asteroid impacts blanket the Earth with cloud cover in a couple of hours, and this takes years to clear. The hydrologic cycle slows to a crawl as solar energy can’t reach the surface. Within days large fauna start to die, and vegetation eaters begin to starve. Predator species eventually run out of food; they may even lose their marbles from the constant darkness.