I agree - but will add that I was doing SEO several years before I gave a toss about Google.
Alta Vista, Yahoo, Lycos, Microsoft Search ... all open to SEO - whether it's backlink based or not.
Whatever metric they use - SEO will exist. As long as their are search engines - people will try and optimize their site/method.approach to marketing to get a better ranking in them - link based or not.
Blind keyword stuffing was all it took 17 years ago (white text on a white background stuffed with relative words and phrases - only seen if you "drag clicked" your mouse across the page)
The "META DATA" was important - and then it's synergy with your own site content.
Then speed and for a while even server type. Sitemaps were the absolute top for almost a year. Optimized site maps that attracted the crawlers and were as complete as possible. Many sites didn't have them (none of this easy "Wordpress site map" plugin back then - you wrote your owm...URL by URL)
Wierd stuff like micro adjustments to a page to "freshen" it worked for a while.
Some people had tools/methods/systems for all this stuff (SEO automation) Keyword relevancy was quite early on the agenda (very simple - but initially very effective)
All of this was years before Google cared about links. All of it was SEO..and all of it could be gamed by those in the know.
Welcome to 1996
Scritty