I think that paper assumes a better understanding than actually exists of the functions/capabilities/rates of cells in the brain.

From Appendix B:

"The typical spiking neuron can be viewed as a point-process channel: It converts a continuous time-varying input, like the synaptic current s(t), into a discrete train of spikes at the output, r = {ti}"

Here are a couple interesting bits from recent research based on a new more detailed measurement mechanism of neurons and signaling that showed two things:

1-The synapses at the end of the axon do not all transmit the signal for each action potential. They found a shifting pattern of outbound synapse activity based on the animal learning new visual input.

2-They found spike timing contained information, down to the single milliseconds level.