not "when the author wrote this, he was trying to say"
And the first interpretation is relevant but wrong, whereas the second interpretation is true but irrelevant.
P.S. It seems that the author meant to say “it is not in general possible to find a given bug by examining the code”, which again is true but not relevant.
I would add that (related to your "maintainability" point) ensuring the code is as simple as possible, and thus much more likely to be "debuggable by review", is a goal of review. Even that won't prevent bugs in the absolute sense, as you rightly say, but it boosts your probabilities.