Suppose it turns out that Oliver lives in a region where conspiracy theories are rife or that he is under the influence of friends who are committed conspiracy theorists. Wouldn’t these be perfectly viable situational, non-character explanations of his beliefs about 9/11? Only up to a point. The fact that Oliver is easily influenced by his friends itself tells us something about his intellectual character.

Anyone who thinks they are not easily influenced by the opinions of friends is more delusional than the hypothetical Oliver. I believe strongly in global warming -- but why? I don't have a significant background in earth science beyond my limited high school science education. I have no truly well-understood argument in favor of my position. It's entirely based on my trust of scientific authority, but there are many authorities that others trust which I don't. For example, I'm very skeptical of many famous, well-regarded economists. Some of them have won Nobel prizes. This skepticism is heavily influenced by arguments I've heard from friends and from other authorities whom I've chosen to trust more.

The point is, nothing in that paragraph is remotely convincing in refuting the idea that people believe in conspiracies because of the peers they associate with. Whether you befriend conspiracy theorists may say something of your intellectual character, but there's no real evidence or compelling argument in this article in favor of the direct causal effect it's advocating for.