Judging by https://developer.chrome.com/docs/extensions/reference/manifest/sandbox, the logic is correct:

A sandboxed page won't have access to extension APIs, or direct access to non-sandboxed pages (it may communicate with them using postMessage()).

For example, here's how to specify that two extension pages are to be served in a sandbox [...]

  "sandbox": {
    "pages": [
      "page1.html",
      "directory/page2.html"
    ]
  },