I think it’s not possible to get that information from the XML report.
However, you can use the Greenbone Management Protocol (GMP) to get that information, for example by get_configs’ output <tasks> (which tells you the tasks using scan config(s)) or vice versa via the get_tasks’ output <config> (which tells you the config used by task(s)).
If you are using alterable tasks, that information may not be accurate for past scans of course.