Sorry if that was confusing, I have edited my post to clarify a bit: That sentence was about a feature specific to JPEG XL, which is that if you have JPEG files you can re-encode then as JPEG XL to save space without any loss of quality. It's nice for peace of mind, you can bulk convert a large archive and you know you still have the same pixels everywhere (later you can even turn them back to JPEG without loss if you want). AVIF cannot do that. Now if you are OK with some loss in the conversion, you can also do much better than 20% with JPEG XL.