It's an HDR image, I think.[0] At the moment, only some browsers show it as brighter—Firefox on macOS, for example, doesn't support HDR images and I had to open the page in Safari to see it for myself.

[0] see, for example, this submission from the front page a month ago: https://news.ycombinator.com/item?id=49402521.

It's an 8-bit PNG of a magenta patch (with a dash of white), but the ICC profile is set to Rec.2020 PQ. So I think the high 255 values of 8-bit magenta/white will get mapped to very top of the PQ curve, which is like 10,000 nits (or whatever brightness your display can muster)