No, DEVONthink is not listed among my 4 apps that have Full Disk Access (which were added into that list automatically by default).
I now manually added DEVONthink into this list, restarted DEVONthink, and then attempted to create an iCloud sync store. But, I get the exact same error response I reported at the beginning of this thread. So I removed DEVONthink from the list.