It seems like Graham here is talking about one specific kind of credential--a university degree--and making conclusions about all kinds of credentials. It's im

It seems like Graham here is talking about one specific kind of credential--a university degree--and making conclusions about all kinds of credentials. It's impractical and inadvisable to "just measure performance" for all kinds of medical practitioners, for example. And credentials can be used to determine factors other than just "performance", like trustworthiness.