why app.getFrontend().addItemToBatch return a bool value???

FrontEnd.addItemToBatch and FrontEnd.addFileToBatch are will return a Batch Item Object according to the description of the this document,

but I test this two API with AME2018, 

FrontEnd.addFileToBatch return a object actually,

but FrontEnd.addItemToBatch return a bool value,

now how could I get encode progress with FrontEnd.addItemToBatch???