That's the insight, I think. "Clean Code" tells you to use maximally pessimistic representation for everything, because everything could be extended in some way in every direction. Meanwhile, in the real world, you likely have a good idea what directions of evolution are possible, and which of them are even useful.

Casey's example shows you that, if you design your code to make use of those assumptions, you'll get absurd performance benefits for little to none loss in readability (and perhaps even a gain!).

Some may ask, "what if you're wrong with your assumptions?". Well, you pay a price then. Worst case, you may need to rip out a module, rethink the theory behind it, and rewrite it from scratch - likely forgoing some of the performance benefits, too. Usually, the price will be much smaller. Either way, it's still better than being maximally pessimistic from the start, and writing software that never had a chance of ever becoming good or fast.