Thank you for contacting Syncfusion support.
We have checked your reported issue in the document and we will correct it in our upcoming Essential Studio 2019 Volume 3 release scheduled in the end of September 2019. Meanwhile we have prepared checkbox selection in ListBox sample for your convenience. Please find the sample in below link,
Could you please check the above sample and get back to us if you need any further assistance on this?
Vinoth Kumar S