Comet or Krakatoa?

Spaceflight Now | Breaking News | Comet blamed for 6th century ‘nuclear winter’

Well, this is interesting. I just read about this incident in Simon Winchester’s Krakatoa and he makes a compelling case for this being the result of a massive volcanic eruption, very likely Krakatoa itself, being the cause of the “nuclear winter”-esque weather. The article above makes no mention of the alternative theories and I wonder what evidence they have that this was a comet instead of a volcano.