Taking the example provided in the article, I don't feel like the new code is meaningfully less complex. In fact, since it added some additional indirection, I could argue it's slightly more complex.
The core code with the nested if statements is something that I would probably refactor in some other way entirely. Maybe by taking advantage of other language features. It is a toy example so it's hard to say but that's part that feels like it needs simplification and untouched in this example.
You could additionally test the three helper functions. But the original tests against ProcessOrder would still be needed for completeness, so they wouldn’t necessarily add much except in an Uncle Bob style, “He who dies with the largest burden of gratuitous micro-tests wins,” sort of way.
Now if I really wanted to make that code easier to test, I’d instead be looking into ways to make the whole thing less stateful. Temporal coupling is much more confusing than if statements.
In agreement with your post, this research that measures cognitive load via EEG and time spent shows that the metrics we use for complexity and readability are only partial matches to what is going on: https://pmc.ncbi.nlm.nih.gov/articles/PMC9942489/