I'm a doctor, though not a diagnostician. Diagnosis is rarely hard - there are some hard cases, but they really mostly aren't. Do you have a persistently elevated blood glucose level? You have diabetes. Do you have consistently high blood pressure? You have hypertension. Etc. It's hardly surprising that computers are just as good as humans at diagnosing diseases that are mostly defined by strict, objective criteria.
What is harder is management - finding the right collection of drugs that will effectively treat a patient's diseases without introducing too many side effects. And what's even harder is anything procedural - we have no computers that can actually do procedures at all. Those aren't what most people think of as "going to the doctor", but it's what most doctors do - either manage disease, or do procedures, both of which are either mostly or severely beyond the ken of computers. Show me a computer that can do something as simple as put in an IV, and I'll be greatly impressed. So many subtleties boil down to "well, I saw something once that looked just like this, and the solution was X..." that it's worth trying X before going on to Y and Z.
My wife is a diagnostician - a neurologist. She sees stuff on a daily basis that would flummox any non-neurologist (really, I barely know what she's talking about half the time, and my peers would be much, much worse at that), let alone a computer. As the old joke goes, it's like being a car mechanic - who has to work on the car while it's doing 70 miles per hour down the highway, with zero downtime acceptable.