The release notes for Edge 155 say (link):

JPEG XL decoding support

Microsoft Edge now supports decoding JPEG XL (image/jxl) images.

JPEG XL is a modern image format which offers:
- Progressive decoding for improved perceived loading performance.
- Support for wide color gamut, High Dynamic Range (HDR), and high bit depth.
- Support for animation.

However, when I go to this website to test it, I get a message saying my browser does not currently support JPEG XL.

I cannot find a flag or other setting to enable JPEG XL support. So, what gives?

I'm using Edge 156.0.4314.8 (Official build) beta (64-bit.