With the new support for Oracle JET 9 in Oracle Visual Builder, it's much simpler to create a multi-row selection enabled table that with checkboxes for row selection. Below is a quick video showing how this is done. This is much simpler compared to the approach you had to take before.
The table automatically adds a selection checkbox column when you set the selection-mode.row="multiple" attribute. There is an on-selected-changed event on the table - and you can attach an action chain to it. The event gets an array of the selected row keys as an input parameter. In the video, I'm simply looping over this array and show a notification with the row key referring to them with the expression - [[ $variables.keys[$current.index] ]]. You can of course use this array for any follow up steps you want to do on the rows.