I feel like instead of saying a photo is "in HDR" (which implies you can view it in HDR), they should be saying the camera can capture HDR. None of the photos in this post are actually HDR for me. (Using Safari on macOS 26.3 with an XDR display) (Edit: Oops. A few of the photos are HDR. Just, not most of the comparisons, which are where I was looking.)
They are in HDR for me - Safari on Macbook Air, 26.2. You must have something misconfigured in your display.
Edit: I actually found that a small fraction of the images do display in HDR: the first cityscape image, the photo of the sunflowers, the second photo of the cityscape right after, and the first photo under "Film Meets HDR". Nothing else in the article is HDR, though.