See thing is I don't think there would be hilarious low odds of Ignition. Kyubey kinda has an idea of what a Spark looks like. If one in a million people have one then Jade's Earth should have around 7,000 people with sparks, more or less depending upon the population numbers.
Combined with the countless Incubators patrolling the planet I wouldn't be surprised if they could track down every last one of them and, though one way or another, force them to Ignite.
Then consider there are at least 103 Earth's under their supervision and the number of potential Walkers jumps into the three quarters of a million range.
Lot's of test subjects for the Incubators and it would all be thanks to Jade.