Im not really sure, but this is my take on trying to guess what it could be from logical reasoning
A thread is a specific type of process. Generally speaking, any process in user mode will not be able to do any Kernel-mode operation.
However, threads have a shared memory system, and they don't know about each other (pipes between threads are a 3rd party communication through an "imaginary" file) and thus there should be no constraint on using the memory within a User-mode thread, even if it is shared with a Kernel-mode thread.
Its impossible to create a thread with Kernel-mode if you are in User-mode, but if you are in Kernel-mode you can change your privileges to be User-mode. Kernel code is usually trusted, and therefore creating a thread in User-space for a Kernel-mode process is probably still considered trusted.