@Hadski
From what the MC already said he is the only "who still LIVE until the end" (cf chapter 7.4), so he wasn't reincarnated.
So the only possibility is what you said earlier, Humanity revert back to medieval era, with all the hints in earlier chapters, it seems to be the case for me at least :
[ul] "the world entered an era dominated by magic" (chapter 7.5, https://mangadex.org/chapter/587345/2 ) , " [/ul]
[ul]"As for my age, I don't remember. I stopped keeping track at around one thousand. (chapter 7.5 https://mangadex.org/chapter/587345/7 ) "[/ul]
[ul]"mechanized civilization" during ancient times (chapter 4.1 https://mangadex.org/chapter/445864/3 )[/ul]
[ul]"About three thousand years ago..." ( chapter 4.1 https://mangadex.org/chapter/445864/5 ) [/ul]
So, for 3 000 years, the Mc has been playing the hero of the world...