Since if for at least the first several blocks any bit pattern can effectively be part of a valid message then the unicity distance has not been reached.
All that is needed is some way to distinguish valid plaintext from random noise, which is easily done for natural languages when the ciphertext is longer than the unicity distance.
This has the advantage of slightly compressing the plaintext, thus raising its unicity distance and also allowing radio operators to complete their transmissions quicker and shut down sooner.
