According to Wikipedia, "first century AD is the most probable guess" for the stele's age. By that time, pronunciation was probably nearer to the modern language than to Ancient Greek, although, of course, not exactly the same: https://en.wikipedia.org/wiki/Koine_Greek_phonology#Popular_...

Even Modern Greek, despite the homogenization effect of modern communications, still has local dialects which sound very different from one another and aren't easily intelligible to speakers of the standard dialect.