I don't think it's at all unusual to look at a race profile, and figure out what kind of gear you need for it. And your estimate of the right gear for a climb will likely vary a bit, between the estimate and reality.

Now I would imagine that most folks would simply think as they were actually doing the climb that they needed to go up or down a cog to maintain their desired cadence without blowing up, rather than think I need to be in the 42/19.

But remember there's a bit of a literary device here where he's trying to explain what's going on to the reader. So you might not think in express terms all the concepts that the voice narrating the book has to state to explain to the reader, even though you're doing the same thing when you ride, just not narrating it to an audience in full sentences.