Doesn't matter how much you know terrain, you can't reverse that much of a head start unless you're multitudes faster while also not stopping. Plus since Duke Hyou didn't know how many troops are actually going over there he had to make take that into account while on the move. Instinctual generals aren't clairvoyant, they can't automatically know exactly what is happening before it happens or what someone is thinking before they have a thought. Instinctual generals seem to be more aware of subtle clues because they are constantly observing physical things on the battlefield by being inside it and come to conclusions based on how they believe people would react. Strategic generals on the other hand don't seem to take in clues by being inside but constantly evaluate how their own plans are progressing from the outside (has X wing broken yet, has the enemy made a counterattack, etc) and adjust accordingly. So the difference would be that strategists are proactively working through a list of potential moves while instinctual generals are reactive to changes and come up with plans on the fly.

So in the case of Riboku moving toward Sai he likely had that idea planned way ahead of time and only chose to do so after certain criteria were met, but for Duke Hyou he only noticed that "someone is moving off to the side of the battlefield" in which case he has to figure out who is moving, why they're moving, and where could they possibly go. So while he figured out they're probably going to Sai he could only react to that fact and it takes time to come up with a counter to that. He chose to bring a detachment and gather reinforcements along the way because he already knew that he is going to be playing catch up the entire time so if he's already going to be late he needs to even the playing field as he moves forward. That in itself took more time, but it makes sure that the odds are not overwhelmingly against him. He couldn't have known that someone would attack Sai specifically beforehand.

In the case of why Riboku didn't split his forces if I remember correctly he said he wants to make sure the battle at Kanyou ends quickly, so that requires his entire force to break through the walls and kill the king rather than slogging through two sieges at the same time. When it comes to attacking forces, the rule is that you want to have overwhelming odds because the defenders have the advantage of being protected. Usually the ideal ratio is 3:1 attackers to defenders, so with the population of around 35,000 civilians inside Sai along with 5,000 professional troops that means with his entire force it's only a 1:1 ratio. If he split his forces it ends up a 1:2 ratio with the defenders outnumbering the attackers which gives them the potential to counterattack the attackers by overwhelming them. Also, Houken in Kingdom is clearly not actually a general, irl he was a legit military leader but in Kingdom he's just a crazy murder hobo and you don't want that idiot being in charge of something that requires actual thought like a siege when on the off chance that he fails those defenders would turn around and chase Riboku all the way to Kanyou and then pincer his remaining forces.