I’ve always thought that was the point of much of the genre (which I have not read so much to be knowledgable on): what do you actually know that you can “invent?”
In Lest Darkness Fall, the protagonist tries to invent a bunch of stuff (clocks, gunpowder), but fails at most of them. He is able to build a still, which requires little skill (if you don’t care too much about the small chances of making people blind), and the printing press, which most intelligent people could work out if they worked on it for a bit.
Another more random example: In one of Terry Pratchett’s Johnny books, one of the kids gets stuck in the 40s, realizes he doesn’t know how to make anything at all, but he does know what products will work, so becomes a very successful investor for people’s seemingly-harebrained ideas.
Anyway, I always thought the fun game was precisely that: what do I know now that I could usefully apply to being stuck in X-era? What would people have been able to invent back then if only they had the knowledge? I can steal the still and press ideas, those are good and I know enough about how to make them to make a good stab at it. Arabic numerals are great, but not really marketable, and my knowledge of math is not good enough for me to get the jump on Newton or Leibniz, although I’m sure if I met any mathematicians, I could give them enough of a description of calculus etc that they could take it and run with it, and maybe I’d get a mention in a footnote… Oooh, maybe windmills, that would be good and simple.
