While the urls are almost impossible to guess, they are still public. I was able to open the link you shared with no form of authentication. So technically files in Glide storage are not protected, especially if the url becomes known and shared with those that should not have access.
There have been requests for more secure storage, but I’m not sure if that’s on the roadmap at this time.
But the point is still the same. If I were able to sign into your app as a user and I happened to own a row in a table that links to that image, then you are relying on me to not obtain and share that url with other people, or leave it open and exposed. If that url ever got unintentionally shared, then it’s still accessible by anybody, regardless if they use the app or not.
I understand your point that Private apps and Row Ownership will help to protect the file, but it’s still a public url. Comparing to Google Drive for example, you can share a url to a file, but still require authentication. That is not the case with Glide storage at this time.