A survey by Barna research group found the top reason given by Americans for the fake news phenomenon is "reader error," more so than made-up stories or factual mistakes in reporting.
I don't understand the construction of this sentence."So" is a pro-form. What does it refer to? Is it "reader error"？ But usually "so" refers to a thing, not a noun.
Is "more... than..." in the same use as:
He is more old than middle aged.