There isn’t really a universally agreed upon definition of “legal tender”, so if El Salvador says bitcoin is legal tender in their country, it arguably just is. Regardless of what that means in practice or the reality on the ground.

(Many people think “legal tender” means merchants are obligated to accept it in trade — as was by-the-letter-of-the-law the case for bitcoin in El Salvador — but that’s not actually the case in most countries.)