No, the jpegcrush functionality is implemented in C as an extension to libjpeg-turbo. But yes, I suppose a shell script would achieve roughly the same result.
So why wasn't this upstreamed to libjpeg-turbo? Why fork at all? libjpeg-turbo is still an actively maintained project after all...
They want to do much more, and with a fork you don't have to justify and discuss every single commit with the original maintainer. If you want to modify heavily something, a fork is the right approach.