For my brother and father it is more like they understand emotions in themselves, and that other people feel emotions similarly, but have trouble identifying them or why they are happening in others because it is harder to take another person's perspective.
Once they know that the person is upset, they empathize and feel bad, so they can in fact understand the person, but they could not guess that there was a problem withoute explicitly spelling it out, and could not derive it from any non-verbal cues.
It's not just the ability itslf, it's the training it provides. Normal children get a near immediate feedback if they guessed emotions correctly. It's like having somebody with you 24/7 telling you how the people around you feel. Those without the ability don't have that luxury.