I think wage slavery refers more correctly to the condition many people find themselves in, where they cannot afford the switching costs to find another employer or educate themselves for another career. Perhaps they have a family to support or have basic needs, thus through market discipline they are forced to work in the same conditions.

In other words, although they are getting paid, they are getting paid just enough to live paycheck-to-paycheck, and they can no more be a free labor market participant than a slave.