I don’t think the arXiv makes any claim to require a minimum level of originality or contribution to the state of the art, beyond “write it up as a paper in full academic prose and it can’t just be a proposal”, especially in a history and overview subsection dedicated to exposition. The authors have a pretty good claim for getting published in an expository journal like, say, the MAA Monthly.

(Also, I don’t think this is any surprise to you, but an NSF proposal requires engagement with the literature, and a statement of contributions.)