Not those kinds of communists. GUE/NGL parties range in self-identification from communist to democratic socialist and are indeed quite large and established, even if they don’t have huge electoral successes in many countries. S&D is generally way more popular, socdems of various intensities. GUE/NGL is proportionally strongest in Greece, Cyprus, Portugal and Ireland (at least by EU election results I can’t be arsed to go through national ones). They’re not the kind of party who would stage a coup and then falsify elections.
The splinter groups I was talking about are the like of the German MLDP who get less than 0.1%, compared to Die Linke which isn’t unaccustomed to double-digit results. Best MLDP result ever was 0.4% in the 2006 Sachsen-Anhalt state elections. Which, of course, fits into their ideology, they believe that capitalism can only be overcome by a revolution and its vanguard. You know the type, in fact I think you’re deeply familiar with it. Occasionally they manage to get a seat on the municipal level. They don’t have a group in the European Parliament because they don’t get in.