While I get why, it bugs me that they have comparison images between jxl and other formats, yet it doesn't actually use jxl, as evidenced by all images displaying correctly on my chrome browser.

It uses jxl if the browser supports it, using <picture>¹.

¹ https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...

This is standard practice. They need to use current lossless formats to display examples to people who don't have the format yet. They are still showing accurate examples of compression artifacts. I'm not sure what else you'd expect them to do.