Short answer is yes, that's pretty much the case.
I can't remember the exact statistic but it's something like 5% of people who apply actually are good enough to get on a course. Of those that get on a ratings course only 70% of those pass. And of those not everyone manages to validate at a unit and actually become an ATCO.
Taking that into consideration, from all the applicants NATS see, only 2-3% will make it. Long odds and a sobering thought.