That's correct, that's the endpoint you should use for exporting the content of files that can't be downloaded in the normal way, like cloud-hosted docs such as Google Docs.
That 'path/restricted_content/' error indicates that the file you're requesting has been flagged as restricted (e.g., for malware, DMCA, etc.). If you believe that the file has been incorrectly flagged, contact support for help:
We love to learn from the educators who use Dropbox. Whether you teach kids, teens, adults or a combination of all three, we want to know what apps and integrations you use with Dropbox to help with teaching. Which of the ones below is your favorite, or most used tool?