Nutrition studies are very frequently awful. The use of questionaires to find out what someone eats is inherently flawed (do you remember how many times you ate brocoli this year?) and leads to biased results. People who care about their health will tend towards vegetarian and vegan diets because they are claimed to be healthy, but they will also be more likely to exercise, not smoke, be wary of alcohol, and so on, reducing the likelihood of cancer through those other mechanisms. Unless you actually randomly assign a diet for a long period of time and control for all of those factors your study is worthless, completely swamped in noise and misattribution. Let's also bot forget that people who are richer and more educated are more likely to be lower in stress and have more resources while also being more likely to go to a plant based diet.
Now take a bung of studies that are too short to see the effect, rely on bad data collection, stratify people by diet while also happening to get richer and more health conscious people in one group, and then bring them all together in one big study. You will find an effect. Is the effect because of the plant based diet or is it because that group is richer, less stressed, drinks and smokes less, works a better job, has a better education, and visits their doctor more frequently? I would expect and effect from the latter half, but the former in not shown by this or any other study thus far.