1) Create a new post
2) Add and e-Learning block and upload a zip file
3) Select the "Insert As: Iframe" option
4) Intercept the request to create a new post and in the body:

POST /index.php?rest_route=%2Fwp%2Fv2%2Fposts%2F236&_locale=user HTTP/2

{"id":236,"content":"<!-- wp:e-learning/block {\"src\":\"/wp-content/uploads/articulate_uploads/__ARCHIVE_NAME__/main.html\",\"href\":\"/wp-content/uploads/articulate_uploads/__ARCHIVE_NAME__/main.html\"} /-->","status":"publish"}

change the `src` to a URL of an external site.
5) Open the new post and see that it loads the content from the original source