SE leaves the question of exactly what impacted the Moon unspecified, referring to it as the "agent". Both narratively and in-story, the question of what the ramifications of the event are is far more significant than its origin. It's not possible to change the past, nor does an understanding (narratively or in-story) have any appreciable impact on what transpires as a consequence.
As others have noted, an 8 MT impactor on the Moon would be a quite minor event. It would likely be visible to terrestrial observers (if on the near-side) and leave a visible crater. Might generate ejecta which itself could enter the Earth's atmosphere over time, though likely with little effect on the ground.
As others have noted, an 8 MT impactor on the Moon would be a quite minor event. It would likely be visible to terrestrial observers (if on the near-side) and leave a visible crater. Might generate ejecta which itself could enter the Earth's atmosphere over time, though likely with little effect on the ground.