I thought NodeJS was single-threaded?

NodeJS is, but neither modules nor backends have to be. It's quite common to spin up a bunch of docker containers that provide databases and backends locally.

This is the point at which the MBA stops being a productive environment. I'm not saying this has to be the case for every project, but if your team uses local dev environments, it's quasi standard procedure at this point.

Right, but the GC was talking about developing JS applications on their computer, so presumably they were referring to compiling frontend JS projects, which multiple cores would not help, I think.

It is but running a single threaded build process with resource contention from chrome, vs code, slack, etc etc will improve with more cores.

If you are using make to call Babel or other JS build tools you can use the jobs flag to parallelize it and again the cores will help.