everything else is merely thought to exist.
Lots of definitional issues that play into all this and have to be clarified in order to avoid the many slippery slopes. Maybe one of the threshold ones is to get a grip on "exist." We tend to think that that which does not exist is of no relevance to us, and is purely imaginary even.
But what matters to us, according to Epicurus, is what we perceive through the senses (and also through the feelings and anticipations (?)).
So if we define "exist" to mean "has a permanent and unchanging nature" then truly nothing "exists" except atoms and void.
But if we define "exist" to mean things we can see, touch, taste, hear, and smell (and maybe what we can perceive through anticipations and the feelings of plain and pleasure) then the world around us absolutely qualifies as "existing" even though it is made up of atoms and void.
So we have this "scientific" vs "common understanding" issue again. It's perfectly proper to define the word "existing" in either way - subatomic scientists can use it one way, and therapists and real-world philosophers can use it another way. But woe to those who mix the meanings, and lead ordinary people to think that they don't really "exist' just because they themselves and human beings don't have a permanent and unchanging nature!
Was Democritus speaking purely scientifically, or was he wandering into "ethics" and suggesting that people shouldn't think in our world that anything really exists excepts atoms and void? Was the slipping down to the slope to Stoicism and thinking that we could through willpower make all our troubles go away by telling ourselves that our bodies and ourselves "aren't real?"
I tend to doubt Democritus himself was doing that, but I strongly suspect that others were making that argument, and Epicurus saw a need to take a stand and affirm to everyone that our lives demand that the things our senses reveal to us do in fact really exist-- and that they are indeed all we have.
So when it is said that "everything else is merely THOUGHT to exist" or "exists by convention" (which implies "consensus?) I can see Epicurus having a huge problem with that perspective as already over the ledge and halfway down the cliff toward radical skepticism and "nihilism."
But right or wrong as to these initial interpretations, all of this shows the huge importance of epistemology and a science of knowledge such as discussed by Epicurus (Herodotus) and Lucretius (mostly book 4).