A “tender” is the name given to a small boat that “tends” to the needs of a larger ship, usually by transporting people or supplies from shore or another ship.
With the word legal referring to a contract, the term “legal tender” means to “tend” to a debt sometime in the future, by way of contract.
Meaning, by using legal tender the debt is never paid, as the “small boat” never left the “shore” to deliver the “money”.
I tried to find the exact words as written in the El Salvador Bitcoin law, but found only the proposition of the law.
It says "Bitcoin is regulated as legal tender" not "Bitcoin is legal tender".
If something is as something else, it's not that thing.
Legal Tender