Extended Backus–Naur Form (EBNF) is a family of metasyntax notations, any of which can be used to express a context-free grammar.
Does it mean that a context-free grammar is a metalanguage of a context-free language, and EBNF is a syntax for the context-free grammar as a metalanguage, and therefore EBNF is a metasyntax for the context-free language as a object language?
A metasyntax describes the allowable structure and composition of phrases and sentences of a metalanguage, which is used to describe either a natural language or a computer programming language.
Some of the widely used formal metalanguages for computer languages are Backus–Naur Form (BNF), Extended Backus–Naur Form (EBNF), Wirth syntax notation (WSN), and Augmented Backus–Naur Form (ABNF).
Does it mean that EBNF is a metalanguage of a context-free grammar being a formal language?
Some relevant definitions:
The syntax of a formal language is its structure, and is specified by a formal grammar of the formal language.
A metalanguage of a formal language is a formal language used to specify the formal language (make statements about the formal language).
The object language of a metalanguage is the language described by that metalanguage.
The syntax of a metalanguage is called a metasyntax of the object language of that metalanguage.