I think what the parent poster meant was if the whole town is filled with service providers, their customers must also be from the same local area. But if there are no other jobs for those customers to work at (to bring outside money into the community), they won't be able to afford those services. Local trade with each other (mechanic selling services to an electrician, the electrician doing work for a roofer, and the roofer putting a new roof on the the mechanic's workshop) falls apart when you don't have money flowing into the area from outside (i.e., building trucks to sell to people in other cities).
Edit: After seeing the downvotes on the parent post, and a couple similar ones, I think people are reading "creating wealth" vs. "not creating wealth" as value judgements on the people in those industries. Whereas in this context I think they mean in the purely economic / accounting sense, similar to "cost centers" and "profit centers" in businesses. Without profit centers, the business would crumble, but without the value that the cost centers bring to the business, the profit centers wouldn't be able to function.
So just like Silicon Valley needs to find a way to not price service workers (janitors, mechanics, etc) out of the area, other places need the same healthy balance of money flowing into the area along with a local economy to distribute that money around.