I think the HDR effect just filter the display with natural color. When i switch from vivid to natural screen mode it is very similar to watching HDR movies i think all they do is balance the color to make it look more natural rather than adjusting lightning and shadow areas
HDR is not just about colors, you take a photo of a window or the sky and you will see the difference, it takes multiple exposures, tone mapping, noice reduction, etc and puts it all into an image. it is super cool. But it is true that the phone does some of this with HDR off, but with HDR on it’s a massive difference. The main difference and selling points in phones nowdays is HDR and other processing methods.