For purpose of discussion, assume that due to bad luck, this asteroid has evaded the detection of all amateur and professional astronomers until about six months from impact. The asteroid is too large to deflect with humanity's current spacefaring capabilities, and the general scientific consensus is that the impact will end all multicellular life on Earth.
What do those six months look like?
at the time I read it I thought it was a nuclear exchange. I recall a series of flashes but I'd have to check the text.
This what I remember (and generally leaned toward) but also... there wasn't really anything else that made me think it was nukes.
But then again, I don't remember The Man and The Boy going into any cities where it would make sense to describe the scene as "completely flattened" or with "shadows burned onto walls".
Thinking about it now, there's descriptions of flashes in the distance but nothing describing a asteroid burning through the atmosphere...