Only because you have a hires screen. It uses srcset to select an appropriately-sized image (my browser loads a 650KB version).

Since when having a hi-res screen mean very fast internet ? My laptop has a higher res than my desk screen and I'm reading this from the crappy airport wifi.

The responsive images standard doesn't try to deliver the best image for your network but the best image for your display. I don't know where things stand regarding browsers sharing network quality information, it's harder and may have privacy implications.

You have control of your browser to improve your experience in poor network situations, you could set it to not load images by default or use your browser's responsive design mode to make it lie about your screen's pixel density.

At exactly what resolution does it begin to make sense to encode a photograph as a PNG instead of a JPEG?

Pretty much never.

how is a 650kb PNG file better when an 80kb JPEG could do the same?