Do they distrust their own coders to the same degree they distrust the processes they sandbox? I suspect the answer to this is "no" but I would like to hear from someone who actually codes there.

I think the truth is more like this: You can write safe C++, just like you can keep a secret. It's just that the odds that you fail go up with the size of the team involved, and it does that by compounding. So by the time you get to the size of the Google Chrome team, the odds of failure are very, very close to 1.