It seems reasonable that they'd have a requirement there's a single file they'll run, maybe even with a predetermined name like ./build, and that's it.
The developer can then juggle all their dependencies and run make/yarn/npm/etc within that. It's really not different from having a CI build script.
And surely they should accommodate controlling the ENV of that build process as well?
Perhaps by doing things like setting NPM_TOKEN for the correct account?
Just like you would with CI?
... except they claim that's too hard.