There's a distinction to be made here, order-0 entropy versus higher order entropy. Order here just means how long a chain of symbols that is considered, where order-0 means just count each symbol at a time, versus eg order-1 where you consider chains of two symbols. Like the other commenter says, JPEGXL can recompress JPEGs losslessly to be around 20% smaller by exploiting higher order patterns (eg via move-to-front, lz77-ish matches, and the MANIAC tree context modeling, versus JPEG's RLE (which is "sort of beyond order-0") combined with Huffman coding (which is order-0))