Because I think we all end up at the same position if we agree that "harm" is very subjective and relative. If we think "harm" can be defined objectively, then we're looking at an absolute standard of justice which I don't think Epicurus would allow.
Broken record that I am, I'm going back to the Greek text of the PDs. There word consistently throughout those PDs to talk about natural justice, the nature of justice, to describe the compact, etc. is βλάπτω.
Principal Doctrines, by Epicurus
Henry George Liddell, Robert Scott, A Greek-English Lexicon, βλάπτω
If Epicurus or the early Epicureans wanted to use a word less "ambiguous" than βλάπτω to convey another meaning, they would have. Whether we think the English translation of "harm" is ambiguous doesn't matter. We need to grapple with why βλάπτω is the word chosen to convey the "basic grasp of justice" throughout those PDs and its connection to what is δίκαιος "just, right, etc."