El Salvador is a failed state about to default on its national debt. It's not a real "government" in the way that most of us would define the term.
91% of El Salvadoreans prefer to use USD, and most stopped using Chivo (the privately-owned, government controlled wallet) after withdrawing their initial deposit.
https://www.elsalvador.com/noticias/negocios/el-salvador-bit...
At least for now, El Salvador has enough cash on hand to pay the $800 million bond payable in February 2023. Current cash foreign reserves are about $3000 million.
I'd like to clarify though, that I'm not an economist, and that the processes required and implications of using those funds to pay the upcoming bonds due are outside my scope of knowledge.
The one that is actually important when it comes to currencies, having something valuable (whether it is a resource, power, political influence, military, finances, etc.) to back the currency up with.
But on the most technical level, you are correct, El Salvador has the ability to back up BTC in the same capacity as a country like Zimbabwe can back up Z$.