No, not really a fork. GraalJS is a new independent from-scratch implementation of JavaScript. It uses a pair of radically different execution and optimisation techniques known as self-specialising-ASTs, and partial evaluation, and runs on the JVM.
But a sibling project, Graal-Node.js, does include vendored code from Node.js in order to be compatible with Node.js applications. That's where the commits come from I'd guess.
Compare:
https://github.com/oracle/graaljs/tree/master/graal-js
https://github.com/oracle/graaljs/tree/master/graal-nodejs