A few websites have demonstrated an effect where you can use an HDR video to add brighter-than-otherwise-possible colors to a UI on certain devices, eg https://notes.dt.in.th/HDRQRCode.

But that's not being used here. Instead, in dark mode the site just sets images to 75% opacity and animates them up to 100% on hover. That's the only place on the page where you see a huge block of pure white, so it looks brighter than everything else (even though the text is also pure white).