Why does the brain need billions of neurons to process 10 bits/s? The stark contrast between these numbers remains unexplained…

Not so much. The stark contrast is the result of a set of stupid premises.

our sensory systems gather data at ∼1⁢0^9 bits/s.

Even In the ‘machine model’ of the brain this article appears to espouse … vastly simplistic as it is … the brain processes 10^9 bits of vision, audio, and kinetic clues which usually results in a limited but highly appropriate response … often in much less than a second … to gymnastic moves or crossing a busy street, driving a jet, performing on a violin, whatever. That response quickly arrives as the result of a model built on long experience of actual reality.

The authors fail to come up with -any- source (credible or not) for that ridiculous 10 bits/s number. Their model is so simplistic it isn’t even worth engaging with.