The difference is that the kraken 'about technology' website [0] gives no useful information as to how they are doing it (I guess it's 'proprietary information'), while this article gives a very detailed description of how to compress jpegs.
In other words: end users use kraken, developers read this article.
I can take a guess at that "proprietary information". Most image optimization sites are just thin wrappers over something like mozjpeg/zopflipng/etc. 1% tech, 99% marketing