I think the details matter. Curtain rods are typically 1-4" from the wall. This product looks like it works best <0.5" [0] from the wall otherwise it's possible the hanger will move around or have a "sagging" look.
Finding curtain rods that are that close is uncommon and limited in styles/options (I didn't see one on a quick amazon/aliexpress search). That's not to say you're wrong, but I don't think it's as simple as you believe it to be.
Are you misunderstanding something? Clothes Railings are "normally" fixed on the ends (to the wall of the cupboard) or from above and can thus, be positioned any distance from the rear wall.
Do an Image Search for "Clothes Railing U Bracket" for example.
Having two sidewalls available such that u brackets work is probably not a good use case for this hanger. Usually it means you're inside a closet?
But I agree you could build/attach small sidewalls or simply blocks and use this u bracket. But we've entered DIY space since it's not a common package and "readily available" as a solved problem.
The ones getting sold on this kickstarter are grooved to keep the hangers at a neat 90 degrees to the wall. With a normal rod the hangers can twist some. Whether or not this is a problem is up to personal taste.
