Is this just an assumption based on writing styles or some such?

A person's writing is identifiable, not just from the style, but the content, philosophy, and mannerisms, grammar, word choice... Read it long enough and you can identify it. Socks is a particularly easy case because his ideology stands him apart, there aren't too many lefty liberals here who have a penchant for posting compromised news sources (WaPo, Rolling Stone, etc) as legit.