「よく知っていますね」「とても詳しいですね」という意味です。すごいという驚きの気持ちが含まれます。

Perhaps a few examples of ～通 will help. ワイン通 = wine savvy, a wine expert ... 歌舞伎の通 = an authority of Kabuki ... 情報通 = a good source of information

I suppose you know quite a lot about it.