My use of "blurry" is because you asserted a clear boundary between user and kernel space. While I agree that this boundary is well-defined, it is indeed "blurred" (made less clear) by the glibc function wrappers and vDSO injected functions. Because the glibc library is a system library and the vDSO is a blob of library code mapped in the kernel. It's not a simple interrupt to context switch and return when complete with state having been updated from "over the fence."

To me the description of a "clear boundary" should avoid the amount of nuance around whether the application's call lands in a library or the kernel's syscall handler. The fact that it doesn't means that the boundary is less clear, or blurry as was the term I adopted here.