[Dwedit]

It has an operation mode where it can losslessly and reversibly compress a JPEG further, and "not a jpeg" wouldn't cover that.

[dragonwriter]

JPEG XL is the thing that makes your JPEG smaller?

[Dwedit]

JPEG XL is basically 4 codecs in one...

* A new lossy image Codec

* A lossless image codec (lossless modular mode)

* An alternative lossy image codec with different kinds of compression artifacts than those typically seen in JPEG (lossy modular mode)

* JPEG packer

Because it includes a JPEG packer, you can use it as such.