I'm not sure exactly what they're getting at here. It's true that the vast majority of people don't think critically about policy, but they clearly do vote based on feelings and those feelings are derived from their current standing.
Well that is sort of the point. They frequently note that their choices are often associated with who was the incumbent at the time when [whatever] happened that swayed their decision. This isn't the only point they are making, but it's a general idea that whatever book they wrote touches on. People don't think that logically, and the fact that they just associate whatever happens with the incumbent president is a clear example of their failure of logic. Sometimes they are actually correct to associate the current situation with the president, but other times they aren't. It's mostly just luck in that respect.