So I have looked into your issue.
I believe this is because your are not defining the site generator to be the one from distill.
There should be
site: distill::distill_website
in the main index.Rmd. Without this, rmarkdown::render_site() will use the default generator which is a classic R Markdown website.
The distill::distill_website custom generator is the one dealing with the content of the _post folder which the default generator does not do.
More on those generator in 10.5 rmarkdown’s site generator | R Markdown: The Definitive Guide
In your site, it has been remove from your main index.Rmd in this commit first day · hardin47/m154-comp-stats@b143147 · GitHub
It is included in the template and you may have removed it by mistake. if you put it back, this should be ok.
I don't think there is any other issue with distill but I'll dig also in the Github issue you posted initially