How does parallel build works?

[xxRockOnxx]

I'm looking into Nuxt's builder code for inspiration on how to speed up and do parallel building for our non-Nuxt project.

I saw the code for parallel runner here: https://github.com/nuxt/nuxt.js/blob/dev/packages/webpack/src/builder.js#L100

If I understand it correctly, that just runs 2 promises, but to quote the webpack docs (https://webpack.js.org/api/node/#compiler-instance) it says:

The API only supports a single concurrent compilation at a time. When using run or watch, call close and wait for it to finish before calling run or watch again. Concurrent compilations will corrupt the output files

Is that not a problem? I tried it myself personally but I really didn't feel like it ran in parallel plus didn't speed things up in any way I think.

I saw the use of thread-loader and memory-fs though which I think will help me.