Coronary heart disease (specifically coronary atherosclerosis) was called by the NIH the "epidemic of the 20th century". Apparently its prevalence rose in the early 20th century, and in the words of the NIH: "This increase was associated with an increase in smoking and dietary changes leading to an increase in serum cholesterol levels."
I'm curious just what changed? Did people really smoke that much more? Or is it that people were smoking more cigarettes instead of pipes or cigars (which would just kill you with throat cancer, like it did with Ulysses S. Grant)? Did food really change that much from the late 19th century, or is it something to do with mass production? Or is this just reflecting increased income and urbanization in the US? Or something else, like a decline in other mortal infectious diseases that allowed people to live long enough (and unhealthily enough) to have heart attacks?
I can get into more detail after work tonight, but as a brief lunchbreak response: the answer to your questions are mostly "Yes, to all of the above, plus we got better at looking for it." As usual, my answer is specific to the US, and probably loosely generalizable to Western Europe.
Smoking rates in America did increase steadily throughout the early 20th century up until the 60s before beginning to drop off following the Surgeon General's report in 1964. Public health professionals and physicians had already been alarmed at increased incidence of CHD for some time, and the Surgeon General's report was something of a galvanizing moment particularly in terms of tobacco as a causative factor. The issue with heart disease is it's very difficult to separate correlation and causation, and was even more so in the past.
The average American diet changed dramatically starting roughly post-WWII: the rise of convenience foods as well as improved economic status for many meant an increase in salt, fat, and sugar intake. Americans both had the means to eat more, and to eat a dramatically different type of food. Did this cause the increase in mortality from heart disease? No, but it certainly contributed.
The average American became less physically active throughout the 20th century: instead of working on a farm or other manual labor, more and more people lived in cities and worked comparatively sedentary jobs. This, too, was certainly a contributory factor.
At the start of the 20th century, smallpox was the only disease to have an effective vaccine. By the end of the century, there was a panel of standardized vaccinations, administered in childhood. My mother, a boomer, had measles and mumps as standard childhood diseases, and luckily had no long-term effects from either, but countless children weren't as lucky. In the rare case when someone gets measles today, we are able to more efficiently treat them and are more likely to have a positive outcome. The decline in deaths from other illnesses is also a contributory factor.
The identification of causes of death as well as improved detection of disease are also factors. In 1902, your cause of death could be "you got sick and died" or "you were 68 and your heart stopped". There were no EKGs or advanced imaging, and our methods of both diagnosing disease and identifying causes of death have improved dramatically. Today we can largely tell what killed someone: back then, it was much more difficult, and the actual cause may well have been overlooked.
In terms of medicine it's genuinely hard to overstate how dramatically things changed in the 20th century in America, from epidemiological surveillance to treatments to prevention to a thousand different potential contributing factors to things like heart disease. Smoking was certainly a very big part of that, but the co-occurring lifestyle changes as well as systemic developments in medicine were likely equal contributors.
I am a little surprised by the way the article you linked to frames the rise in CVD (cardiovacular disease or heart disease and strokes) as an outcome of smoking and diet. I think the article is misleading because it narrowly focuses on heart disease alone and the specific period of 1900-1960. If you step back and think about morbidity (disease patterns) and mortality (causes of death) from about 1850 - 1950 in the US and other industrialized nations, the story makes more sense. By 1900, US had gone through what epidemiologists call "the epidemiological transition." The transition describes the point at which a nation becomes wealthy enough that instead of dying from acute diseases (caused by infections and parasites), the population begins dying from chronic diseases (like cancer, CVD, etc). The transition is caused less by access to medical care and vaccines (although they help) than it is by the establishment of basic public health measures -- clean drinking water, adequate housing, minimum nutritional diets. Basically we live long enough that the diseases of old age become more prevalent. (Interesting side note, we also see an epidemiological transition in our pets' morbidity and mortality with the increased focus on pet health in the late 20th-early 21st century.) So the article you linked to begins its discussion after the epidemiological transition.
Rates of smoking really have decreased significantly in the US since the 1960's or so -- from around 40% of adults to less than 15%. And smoking has a huge impact on rates of CVD and some cancers. The dietary link is more complicated for the reasons you pointed out and also because the impact of diet is modified by exercise and environment. But one factor the article doesn't seem to discuss is the impact of worklife on CVD. There is some really strong research associating working conditions with CVD -- having a job with high stress and low control over the conditions of your labor is predictive of CVD which may explain why working class folks have higher rates of CVD than middle class folks who have higher rates than wealthy people. The rise and subsequent decline in manufacturing and mining in the US may also be at play in trends from 1900 to today. And finally, the health care industry has put tremendous effort into CVD prevention, early detection and treatment since the mid-century, so it makes sense that rates are decreasing over this period.