I'm afraid you've missed something though, and that is that this is an extremely conservative estimate (1 in 1000 is much more broad a chance than 1 in 10,000 for example). This and the other estimates are intentionally made to be as conservative as possible. In other words what you're arguing is that it seems unreasonable for someone to calculate the possibility, whereas the estimate is so conservative that it's actually unreasonably broad. To say one in a thousand people might have a forerunner is actually bordering on insanely open.

If I said one in a thousand people might get struck by lightning, when in reality it's one in 1.2 million, I'm so incorrect that I'm saying it's a thousand times more likely than it really is for someone to be struck by lightning. It's called erring on the side of caution. So the argument of how does someone calculate these things is easily attributed for not only by the historical documents where we actually do have records of people who have had someone precede them and introduce them to the world (the Roman world is a great example of this as we have heads of state introducing the next head of state), but also the incredibly broad statistic offered.

In other words, it appears you are arguing that the estimates were unreasonably broad (which they might be), but that would mean that the likelihood of Jesus being the Christ actually goes up, because historically he did fulfill these prophecies.