So who has said it's to do with anything but the width of the page
You may have implied it yourself when you started bringing up viewports and breakpoints.
But, we know that the 'fold' is at different positions depending on the device used, so if you look at it that way, it does have something to do with the width - the viewport width.
I have been following the process of the lazy loading in SD, and I have tested it and found that it is now working in the same way as the demo in the article found in René's link. Whether or not the first couple of images on a site are set to loading lazy, they do load first. And only the ones in view, and a couple of images that are supposed to come into view next, are being loaded without any scrolling. The best practice is, according to that article again, to give the images a dimension. To begin with, we had to use pixel dimensions and sacrifice the % and VW units, but our CC developers have found a way to solve that, so e.g. width: 100% + max-width: 1200px works fine. It even works without any kind of dimensions.
It would of course be great if some script could detect the viewport and 'place' the fold accordingly.