Just a matter of scale. Every seventh person lives in China.

Or to recontextualize: The article talks about a time span of around 45 years. That's around 1.7 billion trees. Remember, that's an english (short) billion, 1700 million. (In other languages a billion is a million millions, and not just a thousand millions.)

If a worker can plant 20 trees a day and works 200 days a year, that means around half a million people are more than enough to do it. In a country with around 1400 million people, that's 0.035% of the population, or roughly one in 3000 people.

Suddenly, it's not all that crazy anymore.