Did the concept of a "meaning of life" even exist in the modern sense during Epicurus' time? And if not, why should we need it, if the ancient Epicureans got by perfectly fine without it?
That is an excellent question. Even if we can chart the appearance of the phrase "meaning of life" in the English language, how would we identify it in an ancient context.
The closest I can come off the top of my head is "meaning of life" = telos/supreme good/summum bonum. So, of that's the case, there's always been a "meaning" to life, but it seems the more modern idea is that each person is supposed to discover their own meaning? Whereas ancient-ly, the "meaning" was more generalized to humanity at large?
The universe does not "care" about us. The universe is not "looking out for us." The universe is not showing us the way or dropping hints to a "meaning" for our lives. The universe just is. We exist for a brief time in The All (ΤΟ ΠΑΝ) in our corner of this kosmos. Humans appear to have a need to feel more important than they actually are in the grand sweep of things. Now, that doesn't mean we get carte blanche to do as we will. Epicurus' philosophy gives us a grounded perspective: we really don't matter in the most ultimate sense; however, if we want our brief life to be "good," there are some general principles we can use: don't willfully harm others, seek the most pleasurable outcome, have friendships, etc.