JPEG XL can be converted to/from JPEG without any loss of quality. See another commenter where shows a example where doing JPEG -> JPEG XL -> JPEG generates a binary exact copy of the original JPEG.

Yeah, this not means what usually we call backwards compatibility, but allows usage like storing the images as JPEG XL and, on the fly, send a JPEG to clients that can't use it, without any loss of information. WebP can't do that.

But that only works when the JXL has been losslessly converted from a JPEG in the first place, right? So this wouldn’t work for all JXL in practice. (Unless I’ve missed something and this is not the case.)

You could start with relatively good jpegli as a codec and then lossless recompress that with jpeg xl. Naturally some entity (server side, app, content encoding etc.) needs to unpack the jpeg xl jpeg into a usual jpeg before it can be consumed by a legacy system.