So, the common thread of κενός is void, empty, not able to be filled.
The main problem I have is that without some further explanation, the "void and empty" do not in my mind go together logically with "not able to be filled."
The "not able to be filled" carries more meaning than "empty", but simply being void and empty does not (standing alone) because the nature of the atoms and void is that they move around, and considering any particular space at a particular time, there's no necessary reason why that space cannot be filled.
Not sure I am being clear yet, and i am not faulting Epicurus, but i think our English usage of "empty" is probably missing the point, without more added to explain WHY it is not able to be filled. And since the allegation in the first place is apparently that the thing is "not able to be filled" then just saying that it is empty adds little or nothing to the statement.
If something is not able to be filled then the question would be "WHY NOT?"