Node's API stability is a wonderful and many-layered thing. While more and more of the API is moving towards stability in the colloquial "is this going to change between versions?" sense, it's still a mixture.
Those are classifications, but are they definitions? It's still not clear to me that "stability" has much to do with the software performing in a consistent matter. Either way it's a careless choice of wording, I've never encountered anybody who saw the word "unstable" attached to a node.js release and understood that it might refer to the API (see surrounding comments).
See this section of the node docs for a description of the different classifications of 'stability' within the API: https://nodejs.org/api/documentation.html#documentation_stab...