The book goes on to deny slavery or serfdom has ever existed in Chin (to further this lie, the author refers to slaves in China as "maids" and "workers" throughout the first hundred pages, never using the term "slave"), after claiming China invented farming and metallurgy, and all of that is on the first page.
I call people living in rural China in modern times slaves.
Mingong (Chinese: 民工) are migrant workers in the People's Republic of China, who, starting in the last decades of the 20th century, have been travelling from the countryside to the cities to work. It is a recent phase of migration in China.
In a rhetorical sense I'm inclined to agree, but to be more specific, slavery in a more literal and traditional sense existed in China as recently as the 1970's, right out in the open (and goes on underground today).