While i believe for this to be an error by GRRM, there is no precise timeline for Greyjoy rebellion, only that it started in 289 and that it was brief.
It could have started by the end of 289 and lasted for a year? Is that "brief"? I mean it would take at least 2 months to raise bannermen (unless they have those Eastwatch ravens). Second it was said that Robert had help from Wardens of the North and West, Eddard and Tywin. North does not have a fleet, so there are 2 options. They had to travel to King's Landing and board the Royal fleet, then sail along the eastern, southern and western coast of Westeros all the way to the Iron Islands, or they went from Winterfell straight for the Casterly rock or Lannisport, where they joined Crownlands and Westerlands forces and boarded Royal fleet aided by Redwyne fleet. Both would take time, North is mainly boasting infantry (and even if they didnt i dont think they would bring a lot of horses for a island invasion) with 3 miles per hour, 18 hours march a day, they would reach KL in one month. I have no idea how long would it take to sail from KL to Iron Islands, or how long would it take to reach Casterly rock/Lannisport from Winterfell. We dont know how long was fighting part, not even siege of Pyke.
All in all if we assume that it took at least a year from Balon's proclamation until the kneeling part, we could reach late 290. Again, Eddard has 2 ways to go back, go from Casterly rock/Lannisport directly to Winterfell (this is unlikely, there is no direct route/road iirc) or go to the KL then take the ship to White Harbor or go on Kingsroad. If he opted for the second option and went to the KL, he could have witnessed birth of Tommen if it happened early 291.
Fuck this, i thought about this probably more than GRRM did, it was an error.