That conversation very much suggested Shiroe has actually been doing some heavy research during the many, many times we have seen him just sitting behind his desk while the other characters were out partying. We just didn't know what he was doing, but now it's clear he was researching the world quite deeply. I guess much of the world info was also from the times when the game was still a game, and I guess from conversations with the NPC magician or other NPCs that weren't included in the episode. I don't really mind, to be honest. Shiroe is called an archmage, so it makes sense he knows a lot.