My stance regarding calumny is that I would want to believe that Epicurus would not have said what Diogenes Laertius wrote, because Epicurus focused on what the senses perceive. This means that any criticism would have been spoken in this way: X said "xyz" which is incorrect because of such and such Epicurean physics, etc. ---- or X is wearing his toga with red stains of the wine we saw him spill last night (this is just a hypothetical example of specific observations). So he would not have used abstract ideas which are vague categories which pigeon hole a person as forever having the same character (branding someone as a permanent cheat, etc.) and which would be very Aristotilian and much like something that Aristippus of Cyrene would have said.
On the other hand we can't know for sure about all this.