To be fair to lowtech magazine, the author does walk the walk, their office setup is powered off solar panels perched on the window ledges [2].
So I would hesitate to say it was green washing, rather than than just not finding the best compression technique.
[1] https://blindedcyclops.neocities.org/low-tech-image-tests/ga...
[2] https://www.lowtechmagazine.com/2016/05/how-to-go-off-grid-i...
I find it at least a bit difficult to believe that either Low Tech Magazine or Gauthier Roussilhe simply don't know any better about optimizing images for compactness at this point. Surely someone has tweeted advice to them even if they didn't originally know. Maybe that's my own failure of imagination.
They do ask for feedback on improving the energy efficiency of the website but viewing it only as a series of optimisations is a very narrow view. The broader context is much more interesting.
The solar powered website, to me feels like an experiment, so I'm not sure its fair to require it to be 100% 'correct' first time.