Yes, that is getting your site to rank for certain keywords at all and then more niche keywords and the long-tail, long keywords of three to five or even more words. If a site is so badly optimised that it won't even rank then yes it has a poor keyword usage or structure or even mark up issues.
When it comes to competitive seo, i.e. ranking in the top 50, you need to consider a link building strategy that social media is part of. Quality content and outreach are the only ways to achieve that.
It is really not that difficult to optimise your site for keywords, but to rank highly you need evidence your site is any good.
Ever since day 1 of google that has been about link building, for the most part this hasn't changed, only Google has worked very hard to weed out spamming link strategies that will penalise your site. To say otherwise is directly counter to what Google publicly advise themselves. External links from relevant, quality pages is pretty much the highest ranking factor, unless your site is just rubbish and/or you have spam links, although social media has continually become more important. Somebody with a link from the New York Times quoting their site as a source is going to do better than someone with a link from their mum. Somebody with 1000 Facebook shares and no links is going to be better off than someone with no links and no shares.
You might optimise your site for keywords only and get listed somewhere, and organically gain traction over time, but that is a gamble and most people want results sooner than a few months to even years. Google picks up on an exponential increase in external links for sites with a high page rank well, as that is largely how Google is (and always has been) designed to figure out. Social media is almost as important, on average (as quality of sites vary massively), I'd say as (or even more) important.
This is assuming that typically most sites are not as original as they think and there is usually well established content already out there.
It is much easier to rank for something like a brand name that is a made up or unusual combination of words, as that is usually the first goal of establishing a website, or it should be. You can do that with keywords and site structure. But most people want to attract customers around certain subjects as well, that is more challenging.
I think the 80 20 split manny says is about right if you are focusing on technical and content seo, but once your site is keyword optimised, you need to go further, and you can start work on those things from the very beginning by focusing on quantity content people want to share. No matter how good your technical seo is, you can't be competitive if your content doesn't appeal.
Why did Google invested in their own social network ???
Answer: more quality content signals from real people. Better search results. More visitors. More advertisers. Better click through. More profit.
Everything Google does revolves around their core business of Search and ppc advertising, even Android and Chrome.
So today, actually social signals form a large part of determining what real people consider quality content. Keywords alone were the most important factor before Google even existed. And Google is a progressive company, updating their search algorithms as often as every few weeks.
This chart will help a lot.
But this is a lot of detail. You need to focus on quality content targeting specific keywords first as manny says.