Some commercial sites definitely break reader modes on purpose. I think this is more a case of a Safari bug. The site's text content is just served in raw divs inside table rows. The quoted Usenet posts are inside pre elements inside table rows. A div is more of a logical grouping element than a text-containing block element. So Safari's reader is showing the first text-containing block element it finds.

If you want to break reader mode in Safari I guess the secret is to just use horribly mangled HTML. TIL.