V8 has had arrow functions for a long time, but until recently they were not fully spec compliant. That is also why iojs put them behind a flag (separate from the flag that enabled stable harmony features).
Yeah, 'ship' means different things in different contexts. In the context of your original comment, node stable is getting arrow functions within a week of Chrome stable getting them.
In general, io.js has been very good at picking up stable V8 soon after it ships in Chrome. The exception was V8 4.3, which was not picked up because of API compatibility issues.
This is not a problem for Chrome because chrome doesn't expose the V8 C++ API to large body of third party module writers like Node does. It takes time to deal with some API changes.
https://code.google.com/p/v8/issues/detail?id=2700