Comment by d-us-vb

Personally, I would tell you that whatever understanding you gain may still have bugs. Unless your understanding is as complete as a formal treatment of the code, then there may still be bugs in the code due to shared misunderstandings between author and reviewer. The biggest one is both having an incomplete understanding of what a library function does.

So while there may be some overlap, particularly if each person has full understanding of the code's dependencies, in the general case, understanding code and finding bugs are quite different aims.