I started poking at some source files at random in ntoskernel, and it only took a minute to get some "hits" from the ReactOS source:
$ rg InitializeContextThread
./ntoskrnl/ke/thrdobj.c:872: KiInitializeContextThread(Thread,
./ntoskrnl/ke/powerpc/thrdini.c:53:KiInitializeContextThread(IN PKTHREAD Thread,
./ntoskrnl/ke/i386/thrdini.c:92:KiInitializeContextThread(IN PKTHREAD Thread,
./ntoskrnl/ke/amd64/thrdini.c:36:KiInitializeContextThread(IN PKTHREAD Thread,
./ntoskrnl/ke/arm/thrdini.c:53:KiInitializeContextThread(IN PKTHREAD Thread,
./ntoskrnl/include/internal/ke.h:464:KiInitializeContextThread(
$ rg NpxFrame
./ntoskrnl/ke/i386/v86vdm.c:473: PFX_SAVE_AREA NpxFrame;
./ntoskrnl/ke/i386/v86vdm.c:480: NpxFrame = &StackFrame->NpxArea;
./ntoskrnl/ke/i386/v86vdm.c:481: ASSERT((ULONG_PTR)NpxFrame % 16 == 0);
./ntoskrnl/ke/i386/v86vdm.c:485: RtlCopyMemory(KiGetThreadNpxArea(Thread), NpxFrame, sizeof(FX_SAVE_AREA));
./ntoskrnl/ke/i386/v86vdm.c:510: PFX_SAVE_AREA NpxFrame = &StackFrame->NpxArea;
./ntoskrnl/ke/i386/v86vdm.c:512: ASSERT((ULONG_PTR)NpxFrame % 16 == 0);
./ntoskrnl/ke/i386/v86vdm.c:551: RtlCopyMemory(NpxFrame, V86Frame->ThreadStack, sizeof(FX_SAVE_AREA));
Now that said, I have no idea what I'm looking for, or if this is significant. But since we have the source for both, it seems we can do better than speculate w/r/t how much code is similar between them. I know that does not prove copying but I think the author's point can't be blindly dismissed.