How much of this quotation is from Heraclitus and how much is added by Plutarch is disputed. Some accept the whole sentence, others just a word. (One editor even denies that Heraclitus mentioned the Sibyl at all.) Even knowing which of these words Heraclitus actually said will not help much: it is still difficult to know what Heraclitus is saying. Is he attacking the Sibyl for her ecstatic style or he is comparing his own message with hers (both seem strange yet both are divine)?
It is also unclear whether Heraclitus is referring to a specific individual or to an position or type. The Sibyl was involved in ecstatic prophecy and in some way seems to have been connected with the Pythia at Delphi.