When I look at the browser developer console, while I’m in a mobile view (responsive 290px w), the website still uses a large image. Despite various data-src-img viewports declarations. image of the console
Naturally, that triggers properly size images at Google PSI for mobile.
I’m using the iPhone SE preview, the device width is 375px, and as it’s a high-resolution device, the actual resolution would be 375 x 2 = 750px, so it pulls the 768px width image correctly.