> The article claims only Ada has true separation of implementation vs specification (the interface), but as far as I am able to reason, also e.g. JavaScript is perfectly able to define "private" elements (not exported by an ES6 module) while being usable in the module that declares them -- if this isn't "syntactical" (and semantical) separation like what is prescribed to Ada, what is the difference(s) the article tries to point out?

This is false. For example in Ada you can write:

    package Foo
        type Bar is private;
        procedure Initialize (Item : in out Bar); 
    private
        type Bar is record
            Baz : Integer;
            Qux : Float;
        end record;
    end Foo;

Users of the Foo package know there is an opaque type called Bar. They can declare variables of the type, they can use the defined API to operate on it but they cannot reference the implementation defined private members (Baz, Qux) without compile errors. Yes Ada does give you the power and tools to in a very blatantly unsafe and obvious way cast it as another type or as an array of bytes or whatever but if you're doing stuff like that you have already given up.

In JavaScript there are no such protections. For example if you have a module with private class Bar and you export some functions that manipulate it:

    class Bar {
        constructor() {
            this.Baz = 420;
            this.Qux = 1337.69;
        }
    }
    export function Initialize() {
        return new Bar();
    }

In client code you have no issue inspecting and using the private values of that class:

    import { Initialize } from 'module';
    let myBar = Initialize();

    myBar.Baz = 42069; // works just fine
    Object.keys(myBar).forEach(console.log); // you can iterate parameters.
    myBar.Quux = 'Corge'; // add new parameters
    delete myBar.Baz; // I hope no functions rely on this...

Using the private parts of Bar should 100% be a compilation error and even the most broken languages would have it at least be a runtime error. Lmao JS.