“I don’t understand how someone could believe X” typically means that the speaker doesn’t understand how someone could believe in X based on good reasoning. Understanding how stupidity led someone to believe X doesn’t count.
Most people who say: “I don’t understand how someone could believe X” would fail a reverse Turing test that position. They often literally don’t understand how someone comes to believe X.
Most people who say: “I don’t understand how someone could believe X” would fail a reverse Turing test that position. They often literally don’t understand how someone comes to believe X.