Not the author but it seems that the site was made using Quarto [1] which uses pandoc [2] behind the scenes for producing the final output. The pandoc website suggests EPUB is possible.
Author here: you are correct but the EPUB and PDF stuff actually didn’t work for this (it exited while rendering). iirc one of the problems was Quarto didn’t know how to format Polars dataframes
[1] https://quarto.org/docs/get-started/authoring/text-editor.ht...
[2] https://pandoc.org/