That statement assumes that I wrote what I'm working on.

There are more important things to do than to carefully comb over or instrument thousands upon thousands of lines of other people's work (if that's even an option; it's very often impossible eg closed libraries) for the sake avoiding one of the most useful tools a developer has. When you're using a debugger, the number of assumptions that you have to make about what's happening drops to almost zero. If you are making no assumptions without a debugger or ridiculous levels of instrumentation, the software you are working on is of trivial size, full stop.