The chances of that happening to the average person are close to zero.
That’s the whole point. People don’t watch the news to hear “dog bites man” they watch it to hear “man bites dog”.
No one wants to watch a 2-3 hr long movie about someone’s regular Tuesday at the office they want to watch something that doesn’t happen everyday like an adventure, the perfect couple meeting, or the world ending.
You aren’t allowed to / really shouldn’t be say it if you aren’t Black.
Presumably under this notion someone anonymous who says it should be abiding that cultural norm.
Meaning that if an anonymous person is saying it and you expect them to abide that norm it’d be reasonable to assume they’re Black.