Wanda has a connection to her Decrypted even when she isn't in the same place they are...back on page 73 (before the big hiatus) Wanda was able to tell that Ossomer had turned even when she was in the Magic Kingdom.
Information about unit status is not the same thing as information exchange. An overlord senses all of his units, but can't talk to them directly without Thinkamancy (see an early page in book 1, when Stanley senses a loss of spidews unit). The same goes for casters, who create units I guess. Why would anyone use hats for sending reports, if an overlord could transmit all information and orders between his units?
In a war it doesn't matter who's right, only who's left.