It seems your view of scientific evidence is very based on individuals. You're now focusing on focus on the claims of a very specific type not even plant-based diet and using that to disregard the claims of other evidence about heart disease. There have been RCT studies on it, for instance

Nevertheless, several RCTs have examined the effect of vegetarian diets on intermediate risk factors of cardiovascular diseases (Table 1). In a meta-analysis of RCTs, Wang et al. (22) found vegetarian diets to significantly lower blood concentrations of total, LDL, HDL, and non-HDL cholesterol relative to a range of omnivorous control diets. Other meta-analyses have found vegetarian diets to lower blood pressure, enhance weight loss, and improve glycemic control to greater extent than omnivorous comparison diets (23-25). Taken together, the beneficial effects of such diets on established proximal determinants of cardiovascular diseases found in RCTs, and their inverse associations with hard cardiovascular endpoints found in prospective cohort studies provide strong support for the adoption of healthful plant-based diets for cardiovascular disease prevention

https://www.sciencedirect.com/science/article/am/pii/S1050173818300240