Although it might not be very reassuring to people on the outside who treat Google as a monolithic threat, internal controls like this do make insider attacks more difficult, by making them more visible to internal security and audits.

In particular, it sounds like it would be difficult to do it via log analysis alone; it would have to be a more active attack and may require code changes. Either changing code in source control or somehow swapping in a different binary can be pretty noisy (assuming there are good controls over the build and deploy process).

You might compare with having good accounting. You can't just trust people when they say, "we have strict accounting" because they could very well be lying, but nonetheless, not having proper accounting is a red flag.