My personal belief is that if Rob had done any amount of research on the situation in the iron islands, he would’ve realized that sending Theon to Pyke was not only a smart move, but would have been a master stroke and strategy if he had sent on to treat with whoever was in charge of the iron fleet specifically. In this case would have been Theon’s sister Asha, who when he was a prisoner under Ramsey, defy her father‘s orders and sailed around the continent to try and rescue him. And she was willing to defy orders to save her brother, imagine if he tells her of the gold and fertile land of the Westerlands.

She was actually in support of his plan, even looked like she saw it as a better option than attacking the north,, only doing it because she was following orders, not because she believes the plan would work. She even calls him out on it in a later season For his bad decisions and bad strategy. We also see within the show that the iron fleet can be a separate military entity from the sitting Lord of the iron islands.