All four combinations exist in nature: Open source code with plenty of eyeballs, open source software with no eyeballs, closed source software with tons of reviewers, and closed source software with no no reviewers. It'd be interesting to see what the percentage break down is amongst these, but it probably wouldn't be surprising.

It's also another tick in the box as to why pre-checkin code review is so important - bad code is often immortal, and it can be really hard to patch out bugs if people have grown to rely on broken behavior, so it's best not to get yourself in that state to begin with.