[arghwhat]

It isn't a loophole of GPL, it's a feature of the project in question.

If having GPL-licensed code execute non-GPL binary components violates the license, then it would be a license violation to execute any binary that is not GPL-licensed on such an operating system running a GPL-licensed kernel. Such a limitation would be absurd.

From a high-level, project-agnostic perspective, there is no difference between a kernel module and an application binary—they are both external binary blobs with the intention to extend the functionality of the initial project (in this case the Linux kernel). The differences are low-level and kernel-specific (kernel- vs. user-mode, API availability, intended functionality). The GPL does not distinguish between a printer driver or a calculator application.

Another aspect would be the use of a GPL-licensed header file. However, coming to the conclusion that using an untouched header-file (which usually only contains an API surface) from a GPL-licensed project taint code to require GPL-license for compliance would have huge consequences. Many projects would be rendered in violation overnight. This would likely hurt GPL.