Also, quick question: If the planet was the destroyed in the battle where, as Picard says, the Promelians and Menthars "fought to their mutual extinction", who planted the assimilators on the asteroids?
The dialogue in the beginning is curious.
Wes Crusher: "This was the final battle, wasn't it?"
Data: "Neither side intended Orelious IX to be the decisive conflict."
This makes it sound very much as if Orelious IX was but one of the many battle sites in the conflict. Thus, there would have been many other locations where Promellians and Menthars would have dwelt, and we have no idea whether those were reduced to rubble or not.
The dialogue would be very odd if Orelious IX were the shared homeworld of the two species. How could Wesley distinguish "the final battle" by the location if it's the homeworld yet Data's statement makes it clear that this location wasn't supposed to be the one where the conflict would be solved once and for all? And clearly Wesley is distinguishing the battle by the location, as he refers to the asteroid field when saying "this was the final battle" - there is nothing else he can be referring to, really.
Yet the fight at Orelious IX specifically (rather than the overall war) involved the Promellians and the Menthars fighting to their mutual extinction. Taken literally, that would seem to indicate there were no Promellians or Menthars anywhere else, or at least that any such individuals elsewhere had already perished before this particular battle. But we can just as well assume that there were X Promellians and Y Menthars present in the battle of Orelious IX, and that there were zero survivors - after which the remaining Promellians and Menthars elsewhere continued the war, and the Menthars planted those traps for the Promellians. Orelious IX was merely "decisive" in the sense that X and Y were very large numbers and their elimination crippled both sides, making the rest of the war pale in comparison.
Mind you, if we interpret Data's phrase the above way, no statement remains to the effect that either species
would have gone extinct. The Captain of the vessel didn't seem to think he would be the last Promellian in the universe, judging by the message... But if there are
Promellians left in the universe, then Picard really should at least have informed them before blowing up their relic of a ship! (Perhaps he sent them a postcard afterwards?)