You must secure the cement board with screws. You do not want any flex in the tile substrate.
You are supposed to tape all the seams with mesh tape, and mud all seams and screw heads with waterproof joint compound. This is sufficient, even though the screw penetrates the moisture barrier.
After all, on the side of your house, the siding nails or the nails for the stucco wire lath, penetrate the vapor barrier.