I think for me, and it sounds like for this author, the context lost by that abstraction makes it harder to review. In my experience it's easier for me to understand a small block of code, but it's harder to understand how it impacts the system when it's out of context.

For example:

x++;

A very easy piece of code to understand. But who wants x, and what values could they expect? Why do we ++ and under what conditions?

Those effects, again just for me your mileage may vary, tend to get much harder to understand.