I'm pretty sure the quoted sentence is popular meme in the wider BDSM community because it is compatible with the status games wider Western society plays with regards to sexual ethics and practice not because it is true (it may be).

The meme as both principle and practice was developed as a way of defending BDSM from the vanilla public. Employer-employee relationships are a standard part of society and need no such defence.

One might suppose that tautologically, the sentence is true of those relationships which are so conducted, and not in those that are not. But this oversimplifies things. Without the meme in the air, who would think to ask the question, "where does the real power lie?", let alone answer it with "the sub"? But with the idea available, it becomes an option, whether taken or not, for conceptualising and structuring relationships.