Although not anywhere near the status or authority of these two, I think it seems much more likely that the earlier sticky plant surface trapped smaller prey like aphids and absorbed them as they deteriorated. Putting the development of digestive glands first and not last gives the plant a Darwinian reason to continue evolving more efficiently. Speculation beyond this point, but perhaps later, sensory hairs came about to, as described above, prevent any detritus from trying to be absorbed, whereas previously the sticky substance was constantly created. Perhaps this meant that more sticky substance was created to wash the detritus off and keep the section clear for an insect. From this, the use of sensory hairs follows pretty simply to two folds that would close in upon prey. This would allow absorption from two sides at once, leaving the plant open to feed again quickly. After this, the used insect corpse might be washed away by the excretion mentioned above. Then the development of teeth to prevent larger prey from escaping the two folds, et voila!