In my opinion "That’s not so useful, though, if you want to have multiple developers work on the same project." is much more revealing. Has this guy never heard of VCS? If you use a modern VCS it does not make any difference if you split your source into lots of different files or have it all in one. At least not regarding the number of developers working on the project.
Actually a pretty decent part is in JS since most of the standard libraries are purely written in JS. Github claims that nearly 70% of the node repository is in JavaScript.