Why would you prefer to have duplicates in different formats when other formats supported by all are available.

This is a common storage vs bandwidth trade-off.

The bandwidth saving is miniscule by any metric though.

and using one image format (or fewer number of them) increases the CDN's cache hit rate, so it decreases latencies and makes your site load faster.

It's all about convenience for him, Apple's tools generate Jpeg XL so he can't be bothered to resave it for anyone else. He's like "I use Safari and it works fine"

No Apple tools generate JPEG XL by default. You can, of course, export as jxl on newer OSes, since it's built in, but it's not a default. iOS devices still use HEIC as the default "new" image format, but convert to regular JPEG when shared with services which don't support it.