Rav Shlomo Kluger zy”a (Sefer Imrei Shefer) quotes the Zohar Hakadosh as saying that a talmid chochom is comparable to Shabbos. He explains that Shabbos possesses its own holiness. It does not need to be made holy by beis din; rather, it becomes sanctified on its own. The Moadim, however, only become sanctified after they are set by beis din. A true tzadik is like Shabbos in the sense that he is holy on his own and not because other people consider him to be a tzadik.
The 240 men who led their assemblies and opposed Moshe, however, were not genuine tzadikim. If they had been truly righteous men, they would not have fought with Moshe. They were comparable to the holidays in the sense that they got their holiness from another source. They only were honored because others considered them to be leaders, not because they actually were tzadikim.
This is why the pasuk calls them “kiru’ei mo’ed.” They were like a “mo’ed”, holiday, because they were only considered leaders because they were “men of good name” and had good reputations, but not because they themselves deserved it.