If Poe had said something along the lines of ‘Retreat, nothing can stop it now!’, then it would have been clear that Finn was acting out of pure unthinking hatred for the First Order.
“The cannon is charged, it’s a suicide run! All craft, pull away!”
“Retreat, Finn! That’s an order.”
“Finn? It’s too late!”That doesn’t imply a suicide run could not be successful. It might only imply Poe is not willing to sacrifice the few people they have left. Additionally unlike Poe Finn knows the technology, and obviously feels he has a chance of taking out the weapon. So, it’s just a case of the movie providing a poor setup imo.
That is why we get to see the weapons on Finn’s craft crumple. Poe sees it is hopeless, we see it is hopeless, and Finn is on a mission. Rose stops him because he was about to throw his life away for nothing.