Yes, it is possible to create a thumbnail from a secure container. However, in a particular upgrade, perhaps between 12 and 13, Filemaker made a decision that a PDF was no longer an image, but a document, and they DEPRECATED the feature that used to work perfectly, of creating a thumbnail from a pdf in a secure, encrypted, container. Apparently they forgot to remove the ability for it to happen on a batch import, because that is the only way it will work.
If I remember correctly, I created a workaround in one of my solutions that basically dropped the single file into the Temp folder, and batch imported that single file, in order to get the thumbnails we needed from individual files.
Soooo frustrating when they remove features that we depend on. I have a customer who scans hundreds of thousands of files, in order to go paperless, must have the files saved as PDF's, ARE SCANNED IMAGES, but Filemaker decided PDF's are not images.