You will always find someone somewhere picking up a language / framework and thinking it's production ready, from my experience Crystal is not ready and in the example you gave there is absolutly no reason to no use Rust. They needed C binding, and the fact that they started using Crystal years ago when the state was even worse is very worrisom.
I could go on about what's wrong with the language but:
It's not stable, API change all the time, breaking change all the time, cryptic errors, lot of missing basic features, IDE integration etc ...