I think his point is that number by itself is not accurate. For example, I'm in the US. I've been on lots of trains. Every single train I've been on has had more than 17 people on it. So there is absolutely no way that there are only 17 people per year who ride all trains within the US. My guess is that the number is a shortened version of a larger number (e.g. 876 million or billion or whatever it is)
Thing is it doesn’t make sense anyway because if 51 for the UK meant Billion (It certainly doesn’t mean million because there are over a billion passengers per year on the underground. For this to be true each person of the UK would have to take almost 15 trips by rail a week which definitely doesn’t happen.