My google search on the words "KKE" and "sectarianism" yielded a stunning 823,000 results. It's as if there exists some kind of Word feature that inserts the second word after every use of the first: the KKE is "sectarian" with the same phrasal predictability that the sky is "blue", the summer is "hot", or that kittens are "cute".
But then, what does "sectarian" really mean?
In an article entitled "Understanding the Greek Communists" (as in "Understanding the sexual rites of Kiribati", or "Understanding bipolar disorder"), published in the inevitable Jacobin, Nicos Lountos glosses it this way:
The reality is that the KKE has been paying for its sectarianism more than its radicalism. The KKE not only opposes common action with other political forces on the Left, but it’s stood apart from the broader mass movement in recent years.
KKE "sectarianism", then, appears to involve two nominal features: a. the refusal of collaboration with other political parties allegedly on the Left and b. the refusal to participate in the "broader mass movement."