I think that declaration file (kendo.all.d.ts) is missing declaration, where second parameter for append method is not required (based on this example).
append(item: any): kendo.ui.Menu;
append(item: any, referenceItem: string): kendo.ui.Menu;
append(item: any, referenceItem: JQuery): kendo.ui.Menu;
I think that this is the way, how can I add menu item if there are no items yet in the menu?