Guetzli is not appropriate as a general-purpose tool for optimizing website images. That's not really what it's designed for.
I guess I should specify that I'm trying to give practical advice for people who think the linked blog post is instructive. For the vast majority of people, the simple act of thinking about, selecting, and testing the available settings in popular image optimization tools is going to have a far greater effect than the small optimizations (and sometimes big tradeoffs) that might come from cutting-edge stuff like Guetzli.
The reward per effort of going from "not optimizing my images" to "purposefully optimizing my images using common tools" is typically much bigger than the step from the latter to "using the absolute best possible tool for each image."
Guetzli is for minimizing file size at the highest quality levels for JPEGs. Essentially, it's for nice looking photos.
It only goes down to quality 84 and it takes a looonngg time to optimize. As a point of comparison, the author of the linked blog post dropped his JPEG quality to 70 and was happy with it. A JPEG at 70 (or lower), if you're happy with the look, has a good shot to be even smaller than the smallest Guetzli output.
Generally speaking, the easiest gains in JPEG file size will probably come from just dropping the quality down and down in tests, and deciding what you can live with. But if you have to have the best quality, and have plenty of resources/time for encoding, then maybe Guetzli will be a good fit.
I guess I should specify that I'm trying to give practical advice for people who think the linked blog post is instructive. For the vast majority of people, the simple act of thinking about, selecting, and testing the available settings in popular image optimization tools is going to have a far greater effect than the small optimizations (and sometimes big tradeoffs) that might come from cutting-edge stuff like Guetzli.
The reward per effort of going from "not optimizing my images" to "purposefully optimizing my images using common tools" is typically much bigger than the step from the latter to "using the absolute best possible tool for each image."