$ARG_0 variable to `version` hook of `exec` plugin not as implied by documentation

[jwodder]

The documentation for the exec plugin gives the following example:

{
  "plugins": [
    [
      "exec",
      {
        "version": "npm version $ARG_0",
        "publish": "npm publish && git push --tags",
        "afterRelease": "yarn docs && push-dir --dir=docs --branch=gh-pages"
      }
    ]
    // other plugins
  ]
}

This example strongly implies that, when using the version hook via exec, the $ARG_0 envvar will be a valid argument to npm version, i.e., either a version string or a semantic version component name like "minor" or "patch". However, I am finding that it is actually a string of the form {"bump":"minor"}. Is the documentation wrong, or is auto bugged?

I have not checked any other exec hooks to see whether something similar applies to them.

Environment information:

GitHub Actions ubuntu-latest environment, using the latest auto downloaded from https://api.github.com/repos/intuit/auto/releases/latest