"...It remains unclear, they said in a statement, whether the caterpillars digest the plastic with enzymes in their gut, or whether they break the substance down through a process of "mechanical milling"—in which case the plastic would be excreted chemically unchanged." 2017 https://phys.org/news/2017-09-german-scientists-plastic-eating-caterpillars.html ‘A huge step forward.’ Mutant enzyme could vastly improve recycling of plastic bottles By Robert F. ServiceApr. 8, 2020 , 5:45 PM https://www.sciencemag.org/news/202...ould-vastly-improve-recycling-plastic-bottles
To escape the plastic bag she put them in. I might have a different guess if she found them still munching on the bag ... after they had already made holes big enough to escape through. Regarding The experiment: The article doesn't detail the experiment enough to offer a guess.