

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.



Popular Mechanics is US-based, so they use short billions: 1bio = 1 000 000 000.
You are right, long billions (1bio = 1 000 000 000 000) would be a lot more. Though even that would still be possible, but then ~35% of the chinese population would have to be employed in planting trees.