That is far beyond my maths, but from a skim it looks purely theoretical. There is no methods section, with a "Model" section where one might expect it. Is that how you read it?It was remiss of me to omit the reference. Do the math yourself!
https://www.pnas.org/content/108/30/12233
or not
https://www.quantamagazine.org/how-twisted-graphene-became-the-big-thing-in-physics-20190430/
Oh, and I hate that so much science journalism has no references.