It’s ultimately allocated with the system allocater but that does not mean that your DLL function could do a free or realloc on that pointer address as it is not the actual start of the memory buffer but at some offset in a larger memory buffer.
Even if it would, it’s still not safe to use C library memory allocation functions since your DLL linked in C runtime library may be a different one than what LabVIEW was linked with and they work on entirely different memory heaps.