The message wasn’t “hit this exhaust port” but “I’ll explain to you what is going to be unstable in the very core of the death star, and you guys will have to find out a way to make it blow with whatever weapons or skills you may have.”
I’m sure there were other ways to destabilize the reactors, not just that port. Maybe a saboteur getting into the control rooms, or a bomb somewhere else inside.
I think that’s why, in A New Hope, the Empire hadn’t already closed off the port, and doesn’t immediately see what is happening when the rebels do attack. There is a short scene where Tarkin is told “we’ve analyzed their attack, and there is a danger”. Even after the attack began on that very port, it took a team to work out why it might be dangerous!
Overall, I’m worried there might be a movie to explain some of the backstory of Rogue One to explain some incongruities it has. And then another movie to explain some of the incongruities IT has. Then it’ll be prequel movies for incongruity straightening all the way down.