It isn't slavery. It doesn't come close to slavery.
The workers are not owned, they're not captured, and transported thousands of miles, some dying on the way, including children. They're not beaten, abused, lynched, hunted if they try to escape. They're not trafficked. They're not sold. They're not forced to work. They're not children.
They're free to leave. They can withdraw their labour. They can complain to higher authorities. They're paid. They go home at night. They're well fed, and free. They can participate in society's activities. They can take time off when they're ill.