Since I don't know what data or statistics you're relying upon to make the assertion - and I'm not an expert in this field - I certainly can't give you a categorical answer. However, over many years of reading I guess I could say that I have some insight into a way of approaching an answer to the question.

Consider first the truism that "everyone dies of heart failure". That's an elementary fact based on the notion that "when your heart stops beating, then you die." Well, duh. The point is that in our medical diagnosing (and medical examination of corpses), we tend to determine, further than that, "Well why did this person's heart stop beating?" and that leads us to "cause of death".

Most hard-line Communist nations have been notoriously secretive about their leaders' health issues - and about the "true facts" about their various demises, when those occur. For many leaders the country's propaganda machinery is geared to presenting a true "strong man" (in every sense of the word) living and ruling in perfect health. (That also feeds into the nationalistic myths about the terrific genetic stock from which the leader - and his countrymen - come from, the perfection of the nation's health care facilities in general and the genius of its medical establishment, etc.) In the absence of better information, a statement about a leader dying from "heart failure" may be taken at face value, and naïve, unquestioning or gullible correspondents may leap to an assumption that the person died of the types of heart problems that many in the West still suffer: myocardial infarction (heart attack), acute arrhythmia, congestive heart failure, etc. Failing to ascertain exactly what kind of "heart failure" occurred, they just go with that as the "cause of death" and may never learn, whether he simply died from neglect (or ignorance) of other serious health conditions, fear of risky surgery which may have saved his life ... or was perhaps shot in the head by one of the "grieving associates" who now has to «sigh» attempt to pick up the reins, step into his shoes and assume the yoke of command.

There's also the very likely issue that because the medical infrastructure was not "all that" - even for the various Dear Leaders and their ilk - and the medical practitioners may not have been up to the mark of "standard of care" that we assume and demand in the West - and the secrecy about that, as well, because the propaganda ministries would never admit to those facts - then even routine cardiac care and treatment is just not as good in those places as it is elsewhere.

Added to all of that, of course, most of those dictators tended to rule "for life", so where a politician in a democratic republic may be "killed" (figuratively) with bad election results - and then later die of heart disease later in life - the "strong man" in Communist leadership will be in office ... until the day he dies, however that may occur.