You can choose which electricity company provides the amount of power to the grid that you're using. While you don't get "your" electricity, overall you can still affect the carbon balance of the electricity that's produced in your name.
Hetzner is using 100% green hydro and wind power for that, which is as sustainable as any grid-connected company can be.
A lot of EU datacenter providers specifically pick green electricity providers/sources, and pride themselves on it, and use it in advertising their sustainability.
Scaleway in particular are 100% no-CO2 (they have it easy, most of their DCs are in France where it's easy to be fully nuclear+renewable). Hetzner are the same.
Hetzner is using 100% green hydro and wind power for that, which is as sustainable as any grid-connected company can be.