Thank you, Sam Smith.
I uploaded an image, and it was indeed not reduced in size by Imagify. The image served from Google cloud was exactly the same as the original image. When manually uploading the image to imagify.io the size was reduced 31 percent.
So Imagify seems to broken with WP-stateless 2.1.9
I posted the same question to Imagify support. Their answer was that Imagify is not compatible with WP-stateless.
It is strange though that there is an option in WP-stateless to turn on compatibility with Imagify.
I am confused. Can anyone shed light on this?