It could be an OOC conversation.
Alternatively, R2 is the one who had the Nute virus, and the destruction of Cardota and Courtsilius pre-date that incident.
Pete is positing that the Peace Moon is essentially a temporal super-weapon, firing a death ray that travels backwards in time (that doesn't really track with the description of the weapon *firing*, but whatever) so that the interstellar lightshow you see is just normal light that arrived normally, of a system exploding decades ago due to a weapon you just fired right now.
Takodana and the Peace Moon being equidistant from the Hosnian system means that the light from the explosion arrived at both systems at the same time.