I just suggested a game to someone, and they replied asking me "how well" I knew the game. Er, how does one assess the depth of knowledge of a game? Did I miss out on some sort of credentials or what?
What, you've never heard of the "LUDO 9001" standard?
Fear not, nobody's challenging your game-explaining skills. The phrases "Have you actually played it?", "When was your last game?", "How well do you remember it?" and "Are you comfortable explaining it?" are frequently heard near the game library, and are never meant to be rude.
Typically, the true intent of those questions is to figure out how much overhead will be needed until the game actually begins. It's an important detail that impacts play length significantly. The difference between: "I know that game - I can explain it in 5 minutes without even checking the rulebook" and "I know that game - if we sit down with the rules for 45 minutes we'll be good to go" is non-negligible.