It's the latter, ie there's a more fundamental quantum effect going on. The Casimir effect is an effect between two conducting plates separated by only a vacuu

It's the latter, ie there's a more fundamental quantum effect going on. The Casimir effect is an effect between two conducting plates separated by only a vacuum. Strange as it may sound, there is a force generated between the two plates which is dependent on the distance between them. My quick take on the article is that phonons cause a microscopic change the distance between the two plates, which allows transmission of phonons across the vacuum gap.