Drilling at Yucatan reveal, they say, microfossils 300,000 years after
supposed extinction:
"Many believe that the impact shrouded the Earth in dust and debris,
shutting down plant photosynthesis and leading to the rapid demise of most
creatures, from marine microorganisms to dinosaurs."
On a geological time scale, 300,000 years is still fairly rapid. A great
many species may have petered to extinction due to the aftermath (sequelae)
of the impact, rather than the impact itself. I think this is the thrust
behind the K/T impact theory.