It is neither, devDependencies is a redundant distinction for projects where a module bundler is involved, like Webpack, since there's no scenario where you wouldn't want to pull these dependencies.

These are packages published to npm. If you’re bundling all of your dependencies into your package with webpack before publishing it, they should all go in devDependencies, since someone installing the published package doesn’t need them.

You are correct. Got too used to the "just add everything to regular dependencies" shortcut in private projects, which doesn't cut it for published modules.