Seems like the documentation is missing an example on "How to read from an XLS/XLSX file, parse it, and perform actions based on cells' value".
There is, however, an example of writing into the XLS file (https://github.com/wttech/acm/blob/main/ui.content.example/src/main/content/jcr_root/conf/acm/settings/script/manual/example/ACME-203_output-xls.groovy) and a similar, closed issue connected #229).