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I would expect that Capri grille to exhibit ordinary behavior rather than strange behavior, especially since the airfoil sections would tend to have low flow resistance. My guess is that someone misinterpreted what they saw the smoke doing during a series of wind tunnel tests on this Capri. Probably the first application of the streamtube concept to better explain automotive cooling airflows began with Shaub and Charles in 1980. The PopSci article predates this insight into the flow physics by four years. The narrowness of the cooling air streamtube at high speeds could easily have made it look like the Capri grille was doing some self-blocking. The greater the external velocity (relative to the internal flow velocity) the smaller that special spot upstream of the vehicle will be that will actually carry the smoke into the inlet. I took a look at some of my old wind tunnel test footage where I used smoke. The smoke wand would leave about a 3/8-in diameter stream behind it. With a 60-mph wind, even though the frontal inlet of the model was about 3 inches wide and 1 inch tall, most of the time I could not get the inlet to swallow all of the smoke from this relatively small stream. In addition, I could just barely see the smoke stream spreading just a fraction of an inch from the inlet opening plane. My hollow models were connected to the suction side of a high-pressure blower; I could manipulate the internal pressure and velocity over a huge range. It could be that for the Capri, the smoke did not spread until after the plane of the grille. I did not have any trouble interpreting my wind tunnel tests because I was already clued-in on the streamtube concept and my models had no grille at all at the frontal openings. If the guys running the test on the Capri did not have the engine fan going, then the cooling flow streamtube would be really narrow.
Back in ’76, the interaction between the external flow and the internal flow was not understood – things are way different now.
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