That site seems to be pretty buggy. https://coveryourtracks.eff.org/ has more visitors and seems to have a wider collection.

With Safari on macOS I get a score of ~1/8500. Not amazing, but certainly not catastrophic.

It turns out my browser is unique just based on the http_accept header alone. How the heck did we end up in that situation?

It's similar to Spectre. We had more pressing issues back then. As those issues are solved, we moved to more tricky ones which did not get much attention before.