Yes, this is correct. Originally, I tried to make the Marth fight unbeatable, and it seems to still work to bamboozle on the first attempt, but someone figured it out so I decided to add a small third ending in case the player actually managed to beat the boss fight. It is intentionally just a softlock for story reasons. You know, you win but the forces at play have now got everything they wanted so they leave you with nothing.
Viewing post in Marth.exe (Formerly FE SD: Rearmed) comments
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?