The difference between 'damn big clue' vs 'vague, subtle hint' probably depends on how much you know about sickle cell. It's not about a disease being "racialized", it's just plain old statistics. 3/4 of sickle cell cases are in Africa. It's not just a disease that commonly affects people of African descent, it's a diseases where the majority of sufferers are of African descent. Sickle cell is famous for it, even among non medical circles.

Now, you're right, not everyone will know this. But that's not a case of it being a subtle vague hint, it's a case of people missing out on the significance of a damn big clue because they're uninformed on the subject, and if you're uninformed on a subject then everything feels too "subtle" to pick up on.