Controlled studies about lifelong effects of fairly fundamental parts of your life such as diet are quite difficult to do, especially with large enough populations to be statistically significant and without being overly affected by their dropout rate.
Observational studies have significant problems, as I explicitly pointed out in my comment about the study. However, they have some advantages in that they are a lot easier to do for larger populations over a longer time period.
Both of these are valid scientific techniques, to inform us about what is most likely to be a healthy way to eat. There are, of course, other valid techniques, such as studying immediate metabolic effects of ingesting certain foods, and linking those to known risk factors for a variety of conditions. Science is not a single, infallible technique, it is a set of techniques which taken together are intended to give us the best approximation of the truth, though every method can fail in certain ways.
Diet is an especially difficult topic. Human bodies, human behavior, and differences between different people are quite complex. There are a lot of people who are afraid of diseases of affluence or degenerative diseases that you see coming up more often simply due to our longer lifespan. Lots of people are interested in solving these problems, and I think that a lot of people jump to over-simplifying conclusions on these topics. We've seen this played out in the past; a huge over-emphasis on low-fat diets, that seems to have left us unhealthier than ever. Warnings about dietary cholesterol, when it turns out that the cholesterol in our blood stream has very little correlation to cholesterol consumed.
Every time I hear about some new extreme diet that is supposed to work wonders, I get concerned. Low fat, low sodium, low carb, no carb, paleo, high protein, high fat, vegan, raw, gluten free, dairy free and so on. Most of these things seem to focus on some single source of evil, and claim to offer amazing benefits if you follow them. Most of those benefits don't actually play out; despite the huge push for low-fat diets over the 80s and 90s, obesity continued rising at an alarming rate. Despite the huge popularity of Atkins, "paleo", and other low carb diets over the past decade, it has continued to rise.
People keep searching for a silver bullet, and in diets, like much of life, there is no silver bullet. Eat less, exercise more, eat healthier foods like fruits, vegetables, nuts, and whole grains while reducing the amount of sugars and fats, avoiding too much processed food and so on is not some amazing new diet plan that you'll be able to sell some fancy book about, but it's pretty much the actual advice you need to follow if you want to stay healthier.
Anyhow, I see this over and over again, and then I hear from eager people who want to tell me that low-carb, no fasting, no gluten free, no lactose free diets will help them lose weight, prevent cancer, prevent heart disease, prevent Alzheimers, prevent autism or ADHD, do all of the above and more, and there's no risk, they're so healthy, why would anyone not do this? And then I read the literature, and I find that actually, this new diet has quicker effects in the first six months but is about as effective as the last fad diet was after a year, and actually it does have certain dangers, and so on.
Eat less, exercise more, keep it healthy. Beyond that, don't worry too much, and do everything in moderation. Take all dietary science with a grain of salt, since it's a tough field to study the real, long term effects of (20 year studies are considered to be long term studies, but that's only a fraction of a life, meaning that many real long term effects get very little study). Take industrial food with an even larger grain of salt, as it's more likely optimized for shelf life, profitability, or narrow nutrition claims to be able to put some particular label on it, rather than flavor and health. Don't drink soda; I don't care if it's regular or diet, or whether the sugar is corn syrup or cane sugar. Soda is always extra calories that you don't need, or extra sweetness sans calories that confuses your body. And don't drink venti mochafrappucaremelates, or whatever the hell they're called. Don't eat salty snacks on a regular basis, but don't be afraid to put a pinch of salt in your food to make it taste good. Don't have dessert with every meal, but don't be afraid to have a slice of cake at a birthday party.
Anyhow, rant over. I just get upset when I see people getting religious over some new diet fad, which in 10 years will be the diet that the people selling a new diet fad will be talking about in their "you've been lied to for all of these years. Learn how to really stay healthy and lean!"