This is a straw man. Even firm believers in the signaling model like myself grant that schools teach some useful skills. But more importantly, this objection only works against specific kinds of signaling. Yes, if all that school signals is IQ, then a test is a cheap substitute. But what if school signals conscientiousness and/or conformism? Think about it this way: Would you want to hire a high school drop-out with a 150 IQ? Probably not, because you'd immediately think "This guy had the brains to do anything. Why didn't he finish high school? What's wrong with him?!"
It does not suffice to give everyone a test and hire people with the highest scores. Many general-aptitude tests are illegal in the United States because of antidiscrimination laws, but that is not the point. Doing well on a test is no guarantee of perseverance. The signal must be costly and grueling, otherwise it fails to sort out the best job candidates.
From context, Tyler definitely does not seem to be summarizing someone else's view. These thoughts about the signaling model of education are his own, as best as I can tell.
I'm stunned. Tyler's apparent change of mind makes me want to start a Spence Club for believers in the signaling model of education (analogous to Mankiw's Pigou Club) and induct Tyler - or at least his Inner Economist - as the second member.