The only way to get higher search rankings without improving the overall quality of the site is by gaming the search engine's algorithms. If someone is selling that service, overall site improement, they are not doing SEO but traditional design and HCI work. And if they are getting better search rankings without making the site better for users, then they are selling snake oil.
I have to come down on the side of "SEO is bullshit", even though the article in question was mostly content free. It's not that there's nothing to "SEO", just that there's not enough to justify it being a "career". The whole thing smells fishy to me, and I think that the market for it will eventually implode due to all the snake oil being sold. That or the structure will change radically as people who are honest and know their stuff start charging in a different way - perhaps something like getting money for training rather than working directly on a site.
Genuine designers/UI/user experience people are a different breed entirely, who know about making a site nice for people. That's not what "SEO Professionals" sell, though, generally.
Another few disjointed thoughts from my flu-ridden head:
* In terms of comparative advantage, if the thing you are relatively best at is attempting to fiddle with search engine results, as compared to, say, programming, or user interface work, or design work... what's that say?
* ... I had another one a few minutes ago, but my headache got the better of me. Oh, here it is: can't most of this stuff be automated somehow? That would be a legitimate business, creating tools that help people get things right.