I've already talked about it, and many readers will know it either way, but in this context, "isekai" refers to a story in which someone from one world ending up in another. Isekai is a Japanese word that can be translated as "another world," and the fantasy isekai genre in Japan is pretty huge.
It's to the point where the protagonist already knowing the tropes of finding themselves in a fantasy (world because they've read so many other stories about it) is, itself, a trope.
I'm still not sure what would have been the best way to handle the "Nanase recognizes the symbols" limited information in an actual game, but I think part of it is that it would depend on a LOT on individual game context.
The game they're playing has an additional meta context of "this is a story I am gradually sharing with people," and from THAT perspective, at least, I'd say Ellen made the right call.