From that perspective, you've got a finite amount of energy that's being consumed at any point of time and by that logic, you'd be better served to use the solar energy for something else in your household that would've needed the grid, because a cloud provider will almost certainly only need a fraction of the electricity you'd need to keep the static files served.
So, by doing that you've effectively reduced your absolute energy footprint.
But these projects aren't about reducing your energy footprint. They're about having a local webserver that's running on solar energy. Or in the authors words: At its heart, this project is about learning and contributing to a conversation on a greener, local-first future for the web
it's 1000x more helpful to just like only run your big appliances when your grid is using more renewables.
Economies of scale make cloud more efficient for basic sites like this. It is like using street lights vs. every house putting out a few candles.
In addition we love the grid! We want it to get greener. Put the effort into making the grid energy zero carbon, low pollution while still highly available.
Furthermore using that solar to say charge your work laptop and then hosting the site in CF would net use less energy.