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.