The Synchronized Inputs technique depends on an input implementing a special getter-setter pattern documented here. Most of the official Observable Inputs implement it; most community inputs (or plain HTML inputs, like <input />) don’t.
The range slider is from a community notebook by @mootari and doesn’t implement the same pattern. Maybe it could, although I’d have to think about how it’d be different since the value is an array object.
Here’s an example of how you could accomplish the same sort of thing without using set on the rangeSlider directly: