Based on the video output of Xbox One X, the output is clearly derived from the SDR image and is not actually rendered with any kind of high dynamic range. Yes, we can hit a max 500 nits, but this is with the HDR calibration ramped up to the max, artificially boosting peak brightness from what is likely a 100 nit source image. The game's recommended 100 setting for LCD screens would see all of our comparison shots on this page present in a completely monochrome manner, signifying an image that is, effectively, an SDR image.
This is not the first game to present us with what you might call 'fake' HDR - earlier this year the Xbox One port of Nier Automata also pulled a similar trick, effectively transplanting the standard dynamic range image into an 'HDR container'. Once active, your screen registers an HDR output from the console, but it just contains the SDR image within it. All of our tests suggests the same thing is happening in Red Dead 2 on both Xbox and PlayStation systems.