There has never been a recorded insance of an alein microbe devastating earthly life. Even if an alien life form was inoculated into a passing metiorite it would have to survive not only hard radiation if near a sun it would have to survive a hard vacuum and near absolute zero tempratures. If it did survive this it would then have to survive an impact more powerful than any nuclear explosion ever set off as it would be efectively at ground zero. Having done all these things then it would have to be able to combine with earthly life. Meteorites and comets have nearly the same radiation levels of earthly rocks.
Any radiation produced in a catastrophic collision is mainly heat.
As far as alien substances are concerned. Perhaps creationist troll was a bit off the mark. Although turgid pseudoscientific drivel was right on the button. Again, I said nothing about the actual impact itself, but was trying to imply, about the effects on the atmosphere, and its prescence within the biosphere of the earth, esp. All meteorites, esp.