I don't think it is zealotry. They offer features explicitly for Rust programmers, and the major examples of the project's use are by Rust itself.

> Automated testing of Rust code samples

> mdBook is used by the Rust programming language project, and The Rust Programming Language book is another fine example of mdBook in action.

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

> What zealotry?

The headline has been silently edited, hence the confusion. I don't much care for silent editing for just this reason.

I think they'd be best served not using the words "Rust" nor "GitBook" in their marketing, given it lacks most of the features of GitBook (that's probably why it isn't free!); mdBook appears to just be a static site generator.