They can 100% know what was accessed and what wasn’t. This didn’t just happen, it happened in February and their SOC team or an external company would have conducted a full sweep as they’re legally required to disclose what was breached in many of the territories they operate in, which they did four days after the incident took place. I know it’s on trend to hate Reddit right now, but it’s not some one man operation running on a dusty old server in a garage, it’s something like the 20th most visited website on the entire internet, and that comes with certain legal obligations. They know what they’re doing and clearly take this kind of thing seriously.
You don’t have to believe them, but there’s no proof that any user data was breached and they seem to have followed the proper protocols so far. Unless anything else comes out, I’m inclined to believe that they’re telling the truth, or at least not lying.