Hi Ajay,
SAPUI5 application are based on model view controller (MVC) concept. Model holds the data from the backend/database. Per the guide - SAPUI5 SDK - Demo Kit the preferred way to get data from database is OData model through Gateway system.
Also per the mentioned thread - Using UI5 Inbox for Workflow? ,service - /sap/opu/odata/IWPGW/TASKPROCESSING is a odata model
Hence you would need a Netweaver Gateway system to build UI5 applications which interact with ECC workflow.
Hope this helps.