Just as Jews claim exclusivity as holocaust victims of WW II despite that more Germans died and many times more Russians died and it was a holocaust experience for a number of ethnicities, black Americans claim to be the only victims of slavery despite the historical fact that all races have been enslaved usually by their own kind.  Black slavery, for example originated in slave wars between African tribes.

For 40 years, perhaps longer, I have emphasized that in every country with an income tax, everyone who earns is enslaved. In America slavery was officially institutionalized with the income tax in 1913.  People do not comprehend that they are slaves, because they think of slavery as being tied to a location, but in previous times of slavery everyone was tied to location, free men as well as slaves.  Travel was slow and difficult. There were no cars or airplanes.

The historical definition of slavery is unrelated to limits on movement.  The definition of a slave is a person who does not own his own labor.  When a person is captured, enslaved, and sold, the purchaser is purchasing the slave's labor.  Not all of the slave's labor, of course, as part of the slave's labor went for his own maintenance.  The slave's labor over and above what was needed for his maintenance accrued to the slave's owner and constituted the tax on the slave's labor.  

"Free labor" as defined by Karl Marx was labor freed from feudal obligations by the Enclosures.  Prior to the Enclosures which created a labor market and the rise of commodity labor necessary for capitalism, the majority of populations were serfs tied to the land.  The serfs had use rights in the land, and the lords had use rights in the serfs' labor.  The tax rate on the serf's labor could not rise above 30% because technology and thereby labor productivity were low, and serfs could not reproduce if more than 30% of their labor was appropriated.