Well, since Talking About Statistics Is Hard, you have to look at the exact phrasing from the website: it "ensures less than a one in one trillion chance per ye

Well, since Talking About Statistics Is Hard, you have to look at the exact phrasing from the website: it "ensures less than a one in one trillion chance per year of incorrectly flagging a given account". So, each of those billion accounts has a 1:1T chance of false positive. If I remember my stat 101 correctly, that should translate to a 1:1000 chance of having at least 1 false positive on the planet during any given year. (And remember, even a false positive just means that a human reviews your photos, not that you get reported to the police.)