Difference between diversity combining and multiplexing
diversity combining such as maximal ratio combining, the same message is sent over different frequencies


That is not inherently true.

Diversity combining makes use of different channels (not in the "TV channel" sense, but in the "everything that happens to a signal" sense).

Those channels might be created by over different frequencies all carrying the same signal, but often/usually, at the receiver, you use multiple e.g. antennas to observe the same signal.

The different signals are then weighted (often with a complex factor to reflect both phase and amplitude differences), and then added up, is what most commonly is referred to as diversity combining.

You could actually do the same with a single antenna and multiple identical signals mixed to different frequencies. However, that has little to no diversity gain: You actually need $N$ times the bandwidth of the original signal, so you could as well just transmit more data, add more redundancy etc.