Yes, I think that’s probably right, because there is evidence for an impact about the right time:
– eg. the Chicxulub crater was probably formed at about this time (http://en.wikipedia.org/wiki/Chicxulub_crater)
– there is global evidence for impact debris scattered widely at about this time.
Also, we know from some large modern volcanoes that lots of particles in the air can change climate, and so do all the models looking at the effects of massive nuclear wars, which are predicted to cause “nuclear winters”.
So, I think that this idea is probably right – but for now its just a well supported scientific theory – perhaps in the future evidence will be found to show that that idea is wrong, and other ideas are more consistent with the evidence. Some parts of scientific “knowledge” are rather dynamic!