Neither the definition of the roles list, listitem and group nor the definition of aria-level in the specification contains any indication of how nested ARIA lists are created. Is the use of group sufficient or is aria-level required? If there is an element with role=group in role=list, this could be used to group or nest the list entries...? Unfortunately, the specification makes no statement about this.
Therefore I would like to know how

<ul><li>...<ul><li>...</ul></ul>

is marked in ARIA. I would also suggest changing the specification so that it becomes clear.