[Utopia]

They will assume "nuke", and reply accordingly, with their target being their closest enemy.

This kind of asteroid, you can kiss the planet goodbye. But there will be enough people watching it before it impacts to know it was an asteroid. Example: the if Schumacher/Levy comets that impacted on Jupiter as few years back actually impacted on the Earth, you and I and all life on this planet would not be around discussing this.

[Drammach]

Schumacher-Levy was also seen long before it ever struck.
It was a known quantity.

This latest meteor/asteroid was not even noticed until after it had passed earth's orbit.
As a Kiloton-Megaton explosion of "unknown" origin, reaction may be faster than identification.

The possibility of a reactive nuclear strike remains high in such an instance.