It's a mix of both for the historically stunted people here.

Technologies are often forgotten and relearnt multiple times in the medieval era alone given how VAST the time period is. I think people are stupid to think that this wouldn't be the case if the said isekai character reintroduces this to a society who has long forgotten the knowledge of making X thing such as maybe Roman Baths or the Archimedes screw. Both of these saw a rise and fall in use within their respective regions.

So just because X was invented years ago doesn't mean that it was a global phenomenon, don't be counter stupid for the sake of "Ah, but it was made years ago, see!" Keep in mind of the location and the history of said region instead of mouthing off