I don't think this is true. The original specification (ECMA 404) only covers the formal language, i.e. what are valid documents and not what their memory repre

I don't think this is true. The original specification (ECMA 404) only covers the formal language, i.e. what are valid documents and not what their memory representation after deserialisation is. A parser that rejects a number because it has too many digits would not be conforming. rfc8259 is more detailed and it mentions that most parsers will have limited precision and references ieee754 to explain what the expected precision after deserialisation should be for most parsers.