is going to spread debris very very thinly over such a large blast radius, would not be very much debris at all per sq. meter.
Especially if most of the mass was, as it seems with comets, not even rock or gravel, but water and methane ice.
there is no credible mechanism for gathering methane, carburating it to explosive mixes with air, and setting it off to produce the known effects.
Why a person without a geophysical background who is ignoring the evidence of the space-based source of the explosion is making this claim is beyond me.
And you have as yet to acknowledge the fact that space born objects do indeed explode in the atmosphere regularly, which pretty much pulls out one of your main claims.
Ed