I think you're absolutely right that there are nuances such as what you described. More: I imagine there are very few offices/homes that run completely on clean energy - in the most naive formulation of the license, no one could test the software locally!
Likely first you'd need some progressive system (e.g.: no less than 80% now, no less than 100% by 2030).
But I disagree with you on that people in many countries would not be allowed to use the software - you would still be able to use the service (i.e., visit garnix.io, have it build things), or even host it yourself in a different country. Yes, countries with more green energy (rather than exclusively cheap energy) would benefit. But the countries with high green/renewable energy aren't even primarily rich ones [0], so I don't feel too bad about that.
Note that this argument only applies to services such as garnix, where you can still use the service (so it would be a bad license for, for example, a text editor).
[0] https://en.wikipedia.org/wiki/List_of_countries_by_renewable...
