I have no interest in defending Jeremy Corbyn, but in this case at least the situation is far from clear. The Japanese owner of one of the damaged ships, for example, has stated that he does not believe that the ship was damaged by a mine, which goes against the official US position. Not that this necessarily exonerates Iran -- who fired the bullets and shells, then? -- but at the very least it means that the full story is far from understood yet.
It does matter, meanwhile, that US policy, under Trump and his advisers, has sharply turned against Iran -- John Bolton in particular has been seeking regime change there for some time. At the very least, then, they could be forgiven for jumping very quickly to the conclusion that the Iranian government is responsible -- it plays into their narrative, after all, does it not?