Instead of importing, make the backup of the required resources in the source course and restore it inside the target course. The backup / restore process (on contrary to the import process) does transfer the Legacy course files area so the on-demand-migration works.

Using the legacy-file.mbz
#Restore into a new course.
#Confirm Legacy course files shows up, and contains Eris.jpg
#Create a new course
#Restore into current course - Overwrite course config: No
#Confirm Legacy course files shows up, and contains Eris.jpg
#Create a new course
#Restore over current course (deleting first) - Overwrite course config: No
#Confirm Legacy course files shows up, and contains Eris.jpg
#Create a new course
#Do an import from one of the previous courses.
#Confirm Legacy course files shows up, and contains Eris.jpg

Using the no-legacy-file.mbz
#Restore into a new course.
#Confirm Legacy course files does not shows up
#Restore into current course - Overwrite course config: No
#Confirm Legacy course files does not shows up
#Restore over current course (deleting first) - Overwrite course config: No
#Confirm Legacy course files does not shows up
#Do an import from one of the previous courses without legacy files.
#Confirm Legacy course files does not shows up

Test on demand migration of imported course.

1. Restore legacy-file.mbz into a new course.
2. Create a new page activity in the course.
3. Update the page activity in the database:

   update mdl_page set legacyfiles = 2, content='<img src="@@PLUGINFILE@@/Eris.jpg" />' where id = XXX;

4. Do not look at the page activity - go directly to a different course
5. Import the legacy-file course with the page activity.
6. View the page activity and verify that the image is not broken.