yeah the term "wage slave" is certainly an exaggeration that minimizes slavery. I would point out that even Frederick Douglass used the term, but this alone does not legitimize it. It became popular because it's short, to the point, sounds almost poetic (assonance), and gets at the heart of the coercive element of capitalism (you sell your labor power, or you become homeless and starve). But yes, it's certainly not the most nuanced or sensitive thing to say.