Hint for anyone not aware: click the coloured bar beneath the 'commits, branches, releases, ...' buttons to see the breakdown of languages in a GitHub project.
(I agree with you that the README could have made it clearer, but clicking that bar is about the second thing I do when I visit a GitHub repo :) ).
Sorry about that, I definitely wrote the README with a Rust audience in mind. I posted this link to the subreddit for feedback, and really didn't expect someone to post it here.