Aggregative Works and Other Falacies
"Linking forms an aggregative work, which is considered a derivative work under copyright law in the US. "
Please read your copyright law again. If two authors get their works published in an anthology, neither has rights to the other's works.
"If linking isn't forming a derivative work of Qt, then how can the QPL restrict in-house development of undistributed programs? "
Simple, they are restricting the usage of their libraries, some as the GPL does for libraries under it. This has nothing to do with derivation.