SEO is one of those marketing buzzwords that clients are always interested in. The idea that investing in solely in SEO will land you to the top of Google's search results has been around for a while.
Ultimately yes, being "Search Engine Optimised" will improve your rankings, but this is like suggesting only putting your coat on means you are fully dressed. It is one part of a whole.
Google (and other search engines) are much smarter now, they look at lots of factors to see if your ranking is appropriate (bounce rates, quality content, backlinks, mobile optimised etc.). SEO therefore should already be woven into the strands of your marketing, if your marketing efforts are tailored to your target audience.
Today, search engines are far more ingenues in how they rank web sites. By investing the same efforts and resources in to true, relevant user entered content as you would in to this mystical SEO fairy dust, you will get the results you want.