This is sort of like saying "good programming is mostly easy, common-sense techniques that don't change very often", and since programming is a solved problem, anybody willing to pay a lot of money for programmers is clearly getting something hinky done.
Programming is not a solved problem. Neither is marketing. SEO exists at the intersection of programming and marketing. Every site and every business brings their own unique challenges to the mix, and they're not all addressable with "Slap some alt tags on those images and you're good to go", any more than you can solve any programming problem with "Rewrite it... with more AJAX!"
SEO for many startups will involve, among other considerations, "How do we convince our users to create? How do we convince our users to share?" Those are core product/marketing questions for many startups, but they're equally core to the SEO strategy, and they're tricky to get right. For that sort of startup, you could imagine someone looking over their widget's viral activity with the amount of intensity that Zynga spends on its viral channels, because sharing via the widget creates several types of value for the company (via direct traffic, branding, and ranking effects).
A lot of the time, especially with small common issues, the answer is standard and simple and you could have just looked it up yourself. A lot of sites actually just need to pick some decent keywords to target and write titles for the page.
Sometimes, you want a professional to tell you that even though it is mostly just standard stuff because otherwise you don't know if you could be saving tax by doing something else.
Sometimes things are complicated and the difference between a creative (which usually also mean experienced) consultant helps.
BTW, that doesn't mean rely on professionals and don't learn it yourself. Learning a bit of law or accounting might also be a good idea. Certain types of creative solutions are only really going to come from you. Some solutions are best engineered in from the beginning. EG, you might want a lawyer on the founding team of a music sharing startup.
I think the main issue is that most developers/business owners have seen SEO companies charge enormous sums for adding very little value, or using dubious ethical means such as link farming, keyword stuffing, invisible keywords and so on. It's going to take a long time for all of that to wear off.
Bad SEO is easy, good SEO is hard. Just like anything else in life; nothing worth having comes easy.
He wrote "good SEO" meaning ethical or non-sleazy and
"bad SEO (gaming the system)" as sleazy SEO.
Not your re-defining them as "good" as "effective" and "bad" as "incompetent".
Thankfully, I haven't worked at a company that hired a "bad" SEO firm, yet.