I think the implication was that Finn either A) would have been killed before reaching the weapon, or B) wouldn’t have been able to affect the weapon even if he’d made it. So his “sacrifice” wouldn’t have helped anyone at all, it would’ve just been Finn throwing his life away to get some measure of vengeance against the First Order.
It’s something that wasn’t communicated too well, though, assuming that was in fact the intention.