Well it still comes down to a couple of factors. First of all, every single site on the Internet that has at least one word of text has seo whether deliberately or not. And actually, even with no words of text there is still "seo" technically speaking if there is something that can be measured, identified and indexed by a search engine. Then it comes down to "degree of seo". Degree of seo can be measured by "success of seo". Success of seo is measured by "search engine ranking position". So to speak.
A site might never have intentionally or methodically applied "seo" and can still be very popular. External advertising, word of mouth, social marketing and a ton of other ways a site can become "popular." Just one blurb on a national news program can rocket an obscure site that has never even been ranked in a search engine from nonexistence into significant popularity or traffic.

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