What zealotry? Rust isn't mentioned until the last two points of eight:
mdBook is a command line tool to create books with Markdown. It is ideal for creating product or API documentation, tutorials, course materials or anything that requires a clean, easily navigable and customizable presentation.
- Lightweight Markdown syntax helps you focus more on your content
- Integrated search support
- Color syntax highlighting for code blocks for many different languages
- Theme files allow customizing the formatting of the output
- Preprocessors can provide extensions for custom syntax and modifying content
- Backends can render the output to multiple formats
- Written in Rust for speed, safety, and simplicity
- Automated testing of Rust code samples
It mentions Rust because it's written in Rust and used by the Rust project.
In other words, the project wouldn't exist without Rust but