I wonder if they looked into using https://github.com/asaaki/cmark.ex which is an already made Markdown Elixir NIF written in C. No glue code needed since the package already exists.
Back when I was writing Elixir, it's what I used to process Markdown and it was also substantially faster than the native Elixir Markdown library (Earmark).
Author here. I actually was not aware of cmark.ex - thanks for pointing it out.
In this case the code reuse was more important than pure native speed. We already had a Rust library that used pulldown-cmark [1] with some custom tweaks that we wanted to duplicate. Maybe this behavior could have been copied using cmark.ex too (we thought about doing this in pure Elixir, as mentioned in the post), but given how straightforward Rustler made integrating our existing code, this seems like the better choice.