it is not, no. Should it?

It depends. Leave it open for extension? One of the great features of the Jest test runner is how it splits the test runs across multiple processes. For small projects, the overhead isn't worth it but it can help if you have a lot of tests, or slow tests. I'd naively expect to see the same overhead doing Node.js IPC as communicating between web workers. It's good enough the runner is actually split off so you can use it to run other tooling in parallel on large projects, like the linter.