How would they even know how carbon intensive slrpnk.net is? All they can do is measure some page loading speed and maybe do some very general assumptions about how much energy the lemmy-ui needs to be rendered in a browser.

While lightweight websites in terms of browser usage are nice for battery use on mobile devices, it says very little about the overall energy use of a website, or where that energy is sourced from (which makes a big difference for the carbon foot-print).