For UI5 it is right, that you do not need Eclipse with the UI5 development toolkit if you are using SAP Web IDE.
But if you wanna implement e.g. ABAP CDS views or AMDPs you need Eclipse with the ABAP development tools installed. Such objects cannot be created in the ABAP Workbench.
If you do not implement HANA specific objects (e.g. calculation views, stored procedures) than you do not need the HANA Studio aka Eclipse with HANA Tools.
Regards,
Florian