A multiplexed network means that several messages uses the same transmission medium (see below for short version from wikipkedia at the same time.
In telecommunications and computer networks, multiplexing (also known as muxing) is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy, and is now widely applied in communications.
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the high-level communication channel into several low-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.
I have not used CAN but reading this white paper about CAN and Multiplexing they seem to be using using the same definition, if you look at for instance pages 9 and 7 you will note that the CAN uses only one cable that connects all nodes.