I think they're just using Node as a JavaScript runtime. It offers a bunch of libraries, an interpreter (V8), a package manager and so on. Additionally, it's more and more likely to be in your package manager, so it's easy to install.

Basically, it's like having an NLP library for Python--it's not just for the language but for the whole ecosystem. The difference is that JavaScript as a language does not provide stuff like modules, so you have to get that from somewhere else, in this case Node.