Can I use `in` to detect private fields in an instance?

I have questions
1

Since GitHub - tc39/proposal-private-fields-in-in: EcmaScript proposal to provide brand checks without exceptions have been put in stage 4, I have a confusion that whether I can check the private field outside the instance method, which has broken the principle of OOP: Cannot access private members outside a class.

class C {
    #brand;
    isBrandExisted() {
        return #brand in this; // obey the rule
    }
}

const c = new C();
console.log(c.isBrandExisted()); // => true
console.log(#brand in c); // If it don't throw an error, it make me confused
2

You can't use the private field name outside the class' lexical scope. #brand in c is available in exactly the same places as c.#brand.

3

This is correct. You can try it in chrome and Firefox right now.

4

@lightmare @ljharb Thanks for answering.