How can Barristan know that Balon is dead, yet doesn't know that the Greyjoys have attacked the Starks? The former happened a lot later than the latter.
Barristan, as a greenlander, might make the false assumption that Ironborn inheritance works the same as everyone else's, and thus Theon is ruling the Ironborn in the wake of Balon's death. Since Barristan is not going to know about "Theon Turncloak" since that news likely did not get past the neck [Robb would keep his enemies in King's landing from hearing about it, of course, and thus King's Landing can't leak it to Essos], and Barristan is also not going to know where Theon is now or what happened to him, because The Boltons made Theon "disappear", it might still be plausible to Barristan that Theon would take The Ironborn in a pro-Stark direction to help out his buddy Robb, who Barristan ALSO might not know is dead yet, though Robb's demise is rather infamous and seems like the one story that WOULD get around. Maybe Barristan doesn't know that Bran and Rickon are "dead" too and assumes they're a rallying point in the wake of Robb's death if he has heard about it.
But really, Barristan is likely just grasping at straws, trying to find a way for something that makes NO SENSE WHATSOEVER to somehow make sense. Someone from Westeros coming to Barristan's/Team Dany's aid was unlikely enough, but of all the people in Westeros who might, it's the Ironborn
who do so?
And it's a fun sad little kick in the teeth to have Barristan remind us once again how tight Theon and Robb used to be and how much better things at Winterfell were as opposed to how they are now.