As the article indicates, they're similar to ioctl, which is a system call that, roughly speaking, allows arbitrary opaque blobs of data to be passed back and forth between a user-mode -process and a kernel-mode driver. This is intended as a generic mechanism allowing drivers to expose arbitrary functionality to user-space. This enables the implementation of functionality that would not otherwise be possible because it was not foreseen and enabled by the design of the operating system.
Bear in mind that the system call ABI changes slowly and with much difficulty: once a version of a kernel is in production, it can stay in use for a long time; it can take a long time for new functionality to be broadly available, and breaking back-compat with applications compiled against older ABIs is Not Done. Dynamically loadable kernel modules and ioctl-like system calls make it much easier to bring new functionality to all the various kernels running around in the real world.
Given the complexity and rate of change in graphics tech, it makes perfect sense for there to be a general-purpose arbitrary functional-call mechanism for interaction between user-mode and kernel-mode graphics driver components. Microsoft (or the linux graphics subsystem maintainers, etc) just doesn't know enough about Nvidia/AMD's current and future requirements to nail down a more rigorously defined API.
Bear in mind that the system call ABI changes slowly and with much difficulty: once a version of a kernel is in production, it can stay in use for a long time; it can take a long time for new functionality to be broadly available, and breaking back-compat with applications compiled against older ABIs is Not Done. Dynamically loadable kernel modules and ioctl-like system calls make it much easier to bring new functionality to all the various kernels running around in the real world.
Given the complexity and rate of change in graphics tech, it makes perfect sense for there to be a general-purpose arbitrary functional-call mechanism for interaction between user-mode and kernel-mode graphics driver components. Microsoft (or the linux graphics subsystem maintainers, etc) just doesn't know enough about Nvidia/AMD's current and future requirements to nail down a more rigorously defined API.