I believe this is now generally regarded as a misconception, since there are records of the workers being paid. However it can be difficult to tease out the true nature of this work in the historic context, since the way such workers would be paid at the time would likely look like slavery in the modern context (e.g. being paid in grain.) There is also the orthogonal matter of paid/unpaid work, vs forced/unforced work; somebody who is forced to work but is also paid may still be considered a slave.