I figured that was the case, considering what the second ending was. I was expecting a jumpscare in the third ending... but I suppose that it's fitting that there is nothing but the void.
Without Marth, Medeus and the 'interlopers' both have won. But I don't think the former will enjoy his reign much considering what happened after.
That begs the question though: Why is it Hardin whose death will return the world on its tracks? Is his fate truly linked the fate of the world itself? Or maybe Naga-ex-machina shifted this doomed timeline into a mere vision/dream for the dying and atoning Hardin of FE3/FE11?