"responsive app can fetch smaller images" is not really exclusive to jxl though.
For any format you can store an image at multiple resolutions/quality-levels and a responsive app can download the one with the size/quality it wants.
jxl probably saves an incremental amount of storage, which is nice, but storage is not usually the dominant cost of anything. So this is still an edge case.
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"responsive app can fetch smaller images" is not really exclusive to jxl though.
For any format you can store an image at multiple resolutions/quality-levels and a responsive app can download the one with the size/quality it wants.
jxl probably saves an incremental amount of storage, which is nice, but storage is not usually the dominant cost of anything. So this is still an edge case.
That's only true because you took my quote completely out of context.
The point is not needing to store multiple copies of an image. You can simplify the back-end if you serve the same file to all clients.