Well, it's not that common that people just keel over dead and then a medical examiner has to deduce the cause of death from the corpse.

People get sick, they go to the doctor, they get diagnosed, and then they die. Whatever they were last sick from, that's what killed them. A lot of the illnesses on the 1900 list are not really going to have been hard for a 1900 doctor to diagnose. The doctor says you've got the flu or influenza or pneumonia, you probably do.

So the 1900 list is probably pretty accurate.

That's addressed in part in the article. Many "gastrointestinal infections" are thought to have been heart disease. So yes, there's some shift due to changing diagnostic capabilities, as well as definitions of illness.

Not sure of your down-votes, that's the first thing that came to my mind as well.

Edit: Imagine the credibility of a doctor not to at least say: [This is why.]