Software changes are rather easy at Facebook (although that particular tool would surely have a ton of people monitoring it, including quantitative social scientists who don’t monitor a lot of pages). Changing the graph (stored information) is pretty much impossible without a lot of checks. That distinction is key to understand control at Facebook.
Sure they have internal controls so a rogue engineer can't do it. But if Facebook, as a company, decides it wants to mess with your settings, it can, and it does.
This event is an existence proof of that.
Regarding the other point - the fact it took them 5 days to revert the privacy status on weeks' worth of posts of some ~14million users just points to them being thorough and _not_ doing it in a hacky manner. A hackish solution could have been done in an hour or two, most likely.
And also making private anything you legitimately wanted made public, because they couldn't tell the difference.
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