@firosahoge This isn't an isekai though, he's already from this world.
@izayamikaelis The problem is that he ended up being a loner exactly because everyone else treated him like trash for having the crest he had. There were plenty of other people in high level research institutions in his days too, so most of the things he's familiar with from back then probably were passed down until whenever the demons decided to do all of this. It's not clear yet how many years it's been since he reincarnated himself. Should be more than a generation or two though considering how far humanity has fallen.
The first chapter sort of made it sound like his path to being a sage had him living for hundreds of years (unless the translation was just wonky, which is unfortunately fairly likely too), if that was a thing other people attained at all too I'd imagine the time frame since he cast the reincarnation spell is to the tune of thousands if not tens of thousands of years though. Plenty of time to dumb down the masses and hide all the evidence of how great civilization used to be. If he hadn't met that guy who apparently founded the currently dominant sword style before reincarnating, I would've said it's also possible he'd gone into the past before civilization became as advanced as he had experienced.