Yeah, just reading code does little to help me understand how a program works. I have to break it apart and change it and run it. Write some test inputs, run the code under a debugger, and observe the change in behavior when changing inputs.

If that were true, then only the person who wrote the code could ever understand it enough to fix bugs, which is decidedly not true.

I’ll grant you that there are many trivial software defects that can be identified by simply reading the code and making minor changes.

But for architectural issues, you need to be able to articulate how you would have written the code in the first place, once you understand the existing behavior and its problems. That is my interpretation of GP’s comment.