Yes the idea isn't terribly new, but this implementation is significantly more elegant (and ruby-ish) thanks to what looks to be a great design and the capabilities of Crystal. rubyinline is neat but honestly not something I'd seriously consider using in a production project. I need to try out crystalruby before I have an opinion on that, but it looks very promising!

I don’t think the author is claiming that code interop is new.

But this does seem to be the first attempt at interop between crystal and Ruby, which is notable because crystal is effectively a typesafe Ruby.

Crystal existed in 2002?

This seems way more verbose than the project in the post.

Even older, https://metacpan.org/pod/Inline::C (about 1998)