No, it really isn't.

When you drag-and-drop a file from e.g. a file manager into some application, then the DnD events do not contain the file. Instead, they contain something like `text/uri-list` enumerating the file paths. If you sandbox some app, then you need a bit of software to map and translate these paths. If you do not provide such software, then your sandbox does not support DnD.

Well, if you drag-and-drop a file things might be somewhat problematic due to the sandboxing. An app with no access to the file can't read the dropped filename.

From a technical perspective, I understand what you mean. But from a user perspective, 'except for files' is a pretty big caveat. Files are the main thing I drag and drop!

They mean the apps running in flatpak, not the flatpak installer...