"If the alleged comet didn't cause a firestorm, it's much harder to assign it as the cause of the extinctions. Some info here:"

Any impactor air burst can only cause fires in direct line of sight.
So however far the horizon is at say 100,000', all that area could be incinerated. A comet could be fragmented before it enters earth atmosphere- allowing numerous impact areas.

Considering the high velocity of comets, if the comet breaks up in the earth atmosphere this shouldn't affect much in terms of broadening the area. In terms of air burst if one had a very low angle [a trajectory that a slower object might graze atmosphere and return to space] it then seems to me possible that a comet breaking up in the atmosphere could conceivably have significantly wide area affect- but it still wouldn't be continental-scale.

In article you cite, it says, "The shock wave would have spread across the whole continent," said Richard B. Firestone"
Which is true, probably anyone on the continent would feel and hear it. Damage from the shock wave across the continent would not be true but damage from the shock wave would be a larger area than areas incinerated.

So without directly incinerating or tossing Mammoths into the air, such an impactor could affect Mammoths. If the impactor hit in the area in which these animals used in their migration route- all these grazing animals could deprived of hundreds miles of stuff they could eat. And/or large regions of forests could have all the trees knock over- making travel through them impossible.

Directly killing million of herd animals could cause less damage, the blighted area could grow in size, because the animals eat everything on either side of it. The area could more sterile than compared to a natural forest or grass fire and therefore take years to grow back. Offspring might not make through the area, mother feeding their young might not make it through. Predators could flourish in the area.