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

JPEG XL is the thing that makes your JPEG smaller?

	▲	Dwedit 2 hours ago \| parent [-]
		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.