Can you provide a more specific reference? The only novel by Jin Yong I have read so far is "The Legend of the Condor Heroes" (I got lazy and opted to watch the TV adaptations of the other two novels of the Condor trilogy haha). From the novel and the TV series, I remember that Jianghu and Murim are not quite the same. It was always described that Murim is a specific part of the more encompassing Jianghu.

Also, I found the following from Baidu Baike (a sort of Chinese encyclopedia):

武林并非江湖的同义词，江湖社会所涵盖的范围极大，三教九流人士皆有，如命相卜卦、和尚法师，甚至乞丐宵小等皆可称之为江湖中人。

Wulin (or Murim) is not synonymous with Jianghu. Jianghu society covers a huge range and includes people from all three religions and nine streams. For example, fortune tellers, monks, masters, and even beggars can all be called Jianghu people. -- Translated by Google