Many things are "greater than the sum of their parts" and have emergent qualities that absolutely exist, even if those qualities do not exist in-and-of-themselves.
And a key issue is focusing in on what is mean by "absolutely exist."
I've taken to focusing on those things that have "an eternal and unchanging existence" (atoms, void, the universe as a whole) vs those things that do not -- things that can change over time.
But that's only one way to look at it, and I suspect it really means "real to us" in the sense that Epicurus is saying that if our senses, anticipations, and feelings, register it, then we should consider it to be "real." just don't make the mistake of thinking that everything that they register has an eternal unchanging existence, or that everything is equally significant to us.