If you only have acces to the file/folder via a shared link, then you are correct, it is not possible to use the get_thumbnail endpoint with a shared link URL to generate a thumbnail. Nor is there an alternative way to generate a thumbnail.
A feature request already exists to include a preview/thumbnail URL for a shared_link, I have added your additional vote for this feature to that request.
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?