Nothing about richness or leanness is implied or enforced by using Markdown for text, it's about everything else around it. On a statically generated page, you do it in Javascript, and render client-side if needed w/ data prepopulated in static objects. The only thing you can't do with a static page that ingests text as RST or Markdown or any other markup language, is something that requires dynamism with a backend database.
This is trying to blame Markdown and statically generated sites for leanness, but minimalism is a stylistic aesthetic choice people made that was a trend for awhile, it's not a requirement of the technology. The technology is just a tool.