The transmit power budget allowed by the standard is shared with the various antennas at least. Even if a given transmission uses the maximum power to maximize

The transmit power budget allowed by the standard is shared with the various antennas at least. Even if a given transmission uses the maximum power to maximize the modulation and coding scheme used BF will at least reduce the interference to other AP and devices, so it's still has a positive impact on interference, which is reduced.