I only skimmed the paper but it doesn’t see particularly dense, mostly just relying on college math?

It's 50 pages and cites this other paper in the same repo:

OpenAI. An explicit power saving for the exact discrete Fourier transform.

Here's a random excerpt:

8.3 The middle transform and the final permutation The factor QFt in (35) can be computed from a cyclic convolution and two pointwise phase multiplications. The chirp identity below performs the frequency change in Q without applying Q as a separate permutation of the array. The second identity shows how the retained source permutation R cancels when computing a convolution. Here ∗ denotes cyclic convolution on the product of the coordinate groups and a dot denotes coordinatewise multiplication.