The key reason why people don't get SEO benefits from social media

That makes sense as a bottom-line statement. But the contact type matters to people receiving the contact. I deal with people in 2015 who will not accept email OR social media or Skype. In order to deal with them, you have to pick up the phone. Or even meet in person- There are people who want a coffee meeting or drinks at a convention.. or something in-person to do any business with you, It depends.

I think "writeaway" makes a couple points that might be generalized but still hold true in a lot of cases. Social media is "personal" for a lot of people, in a way that email isn't. For them a tweet is close to a phone call. Or a Facebook message is the voicemail that sets up the business meeting. It depends.

I think the overall takeaway is not "You aren't doing effective SEO because you don't do social" but "IF you are going to use social, you want to do these things. Or, this is a way you can use social effectively to create relationships that lead to links." It might not be fair to lump SEO performers into a group of social-media ignorant folks. As you say, some people don't think they need it. To me, I think in 2015 it's a requirement for most brands and very easy/cheap to use. At the same time you are correct that many don't. There are also big-timers who don't have public emails or even identities.