[RightWhale]

"The duration of the fireball was only four-tenths of a second,"

Why is there a fireball? A flash due to ionization as kinetic energy is transferred seems likely, but a fireball would seem to have substantial duration.

[null and void]

It takes a little time for incandescent dust, sand and gravel to cool in a vacuum.

[steve-b]

I assume they're referring to the rapidly dissipating cloud of vaporized meteoroid/moonrock as a "fireball". It wouldn't behave the same as a fireball in atmo, of course.