Well, you can't just add some fancy controls to the toolbar. I don't know how Control.Slider looks like internally, but the documentation for Toolbar.add() says:
If arg is a Toolbar.Button, it is added. If arg is a string, it is wrapped in a ytb-text element and added unless the text is "separator" in which case a separator is added. Otherwise, it is assumed the element is an HTMLElement and it is added directly.
In your case it's neither Toolbar.Button, nor a string. So the only thing left is HTMLElement, which is a raw DOM-element. Does Control.Slider expose some kind of underlying dom property?