Interesting. I don't have a chance to mess with the app, but I wonder if it works because it's syncing individual kdbx files that the user gives access to one at a time rather than a whole directory (including subdirectories). Logseq and Obsidian, for example, would require syncing an entire directory along with subdirectories. My assumption is that they work with iCloud because they're given their own app-specific folder to do as they wish.

If that is the limitation, I wonder if they could get around that by using some type of bundle or archive. Regardless, thanks for the additional information.