I'm not sure why you would say it's not really HDR. It is. It's the base standard, the others use the same 10bit dynamic colors range. The 400/500/600 just refer to the maximum brightness of the display, something that may be a benefit but is not related to whether or not something is "true" HDR. HDR10 is true HDR. All HDR600 etc are also HDR10 with the added benefit of increased brightness.
HDR600 etc are not just a maximum required brightness but also a minimum required black value. That gives you an actual dynamic range; 10bit is not a range, it's a resolution.
