Is time-division multiplexing really a multiplexing method or more of a protocol?
Time-division multiplexing is a protocol. Needs some hardware to process the packets received by the intermediate hardware. Also, the source and the final receiver and all other systems in the chain must adhere to the protocol.

Space division multiplexing is facilitated by the hardware and doesn't demand the receiver/source to be 100% adhere to the protocol. Whereas for time-division multiplexing, all systems in a network must be capable enough to adhere to the protocol. CDMA is the best example where the phone must be capable to adhere to the protocol.