Maybe you're not talking about the same thing Wahern is talking about.

Question: Let's say I know how to snake a sewer line. This is the kind of thing a handy home-owner would be able to rent the equipment for at Home Depot and do themselves if they were so inclined. That's literally the only plumbing skill I know. Can I offer that skill for sale on the open market to customers without first getting the permission of the state (certification on completion of apprenticeship) or an employer who offers me an apprenticeship?

Wahern is clearly saying that would not be possible. So that means that a poor person must get the permission of a wealthy plumbing business owner in order to sell their skills on the open market.

I'd say yes. But I guess you would have problems getting clients to trust you. After all if e.g. some "random guy" messes around with your eletrical cables your house could catch fire... Or water could leak into your walls...