First off, 46% of hiring managers still think it is either "very important" or "somewhat important" where the person went to school. That's nearly half. So it might not be the most important thing, but it is still an important consideration, especially when dealing with a lot of candidates may be similar.

The second problem is that it's a survey, which means that its susceptible to the kinds of implicit biases that people may not even be aware of, or that they be subconsciously fighting against. It's not measuring what hiring managers actually do, but what hiring managers say they do, which can be two different things. If they're answering a survey, they could be motivated to believe that they look at something objective, like performance, because that reflects well on their ability to be an effective hiring manager. However, its harder for them to measure their own preconceptions and biases towards a more prestigious university.

If a hiring manager is faced with the nearly identical candidates, and the primary difference is the school, he might go with the more prestigious school, and then plausibly claim that he was going with the candidate that had better knowledge or experience. That's the nature of the prestige of some schools; it's not that a hiring managers think that two students with the similar credentials are equal, and one just happened to go to a better school. It's that they believe that the student who went to a better school is smarter and better qualified and has more knowledge in the field.