Node uses the V8 C++ APIs to add additional functions and objects to that runtime.
So Node has a specific JS runtime environment, just as browsers have a specific JS runtime environment (and Workers have another), etc.
All of these environments are extending the runtime environment provided by their underlying JS runtime, they're not all providing their own stdlib implementations, they're not providing their own implementations of arrays, objects, global object, etc.
V8 provides a JS runtime.
Node uses the V8 C++ APIs to add additional functions and objects to that runtime.
So Node has a specific JS runtime environment, just as browsers have a specific JS runtime environment (and Workers have another), etc.
All of these environments are extending the runtime environment provided by their underlying JS runtime, they're not all providing their own stdlib implementations, they're not providing their own implementations of arrays, objects, global object, etc.
It is but the Node developers developers didn't roll their own JavaScript runtime.