Yes this tool is pretty crappy. Gave me same kind of advice for my nojs, no img personal website. I do agree with the website's manifest though.
I have a nice alternative benchmark for sustainability: does it load fast on firefox on my xiaomi redmi note 4? Yes? Then it's probably sustainable web design.
Probably a reasonably decent benchmark, given its age and specs.
I think the only reasonable step "up" (or down, specs wise) would be acceptable performance on $30 KaiOS devices, to maximize accessibility to visitors from developing countries, but that's about it?
