What they're saying is that you can take a JPEG compressed image, decompress to raw pixels, and then recompress with JPEG more efficiently (if you're careful, no specifics on how this is done), and you save space.
That's why they mention that they're doing it very carefully, because you've got to make sure that when you decompress the new optimized image that it pixel for pixel matches the original decompressed image.
What they're saying is that you can take a JPEG compressed image, decompress to raw pixels, and then recompress with JPEG more efficiently (if you're careful, no specifics on how this is done), and you save space.
That's why they mention that they're doing it very carefully, because you've got to make sure that when you decompress the new optimized image that it pixel for pixel matches the original decompressed image.