I believe the point here is that they have sealed to the edges of the rafters which creates a seal between the roof area and the insulation below. There is no seat at the top and bottom so it will still allow airflow from the soffit to the top vent, just like the leaky plastic troughs.
Keeping air from migrating up through the rafter bays will significantly improve the general sealing of the house. Keep in mind that the relative air pressure (due to the temperature differential) at the top of the house is quite a bit higher than ambient outside the house, doing this greatly reduces the migration of air through the structure.
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