The designator all describes all notes in the current TBX. 'all' is assumed as a value by some group-based operators if no explicit group name is supplied.
It appears this is explicitly supported for group-based work in actions & rules, but not for group-based export codes.
Designators don't allow offset references as with attributes. Thus $Name("Some note") works but children("Some note") doesn't. If an offset designator is needed use find() instead.