15% of 38.25 million (the population of Canada) is not 600,000. If you're saying that 15% of Canadians identify as neurodiverse, then you've got a whole lot of people you haven't quite accounted for. 600,000 would be around 2%, which I think is a reasonable amount of people.

'half the world has X and half the world has Y, and the other half has X and Y' is a form of hyperbole. It's designed by the way it is written to be ridiculous. For one thing, you cannot have three halves. The hyperbole is there, part stylistically and partly to suggest that perhaps the gap between 2% and 15% is quite a big one.