Quoting the relevant passage in translation, 8:37ff
When the barbarian assailants drew near and were in sight of the place, the Prophet, who was named Aceratus, beheld, in front of the temple, a portion of the sacred armour, which it was not lawful for any mortal hand to touch, lying upon the ground, removed from the inner shrine where it was wont to hang. Then went he and told the prodigy to the Delphians who had remained behind. Meanwhile the enemy pressed forward briskly, and had reached the shrine of Minerva Pronaia, when they were overtaken by other prodigies still more wonderful than the first. Truly it was marvel enough, when warlike harness was seen lying outside the temple, removed there by no power but its own; what followed, however, exceeded in strangeness all prodigies that had ever before been seen. The barbarians had just reached in their advance the chapel of Minerva Pronaia, when a storm of thunder burst suddenly over their heads- at the same time two crags split off from Mount Parnassus, and rolled down upon them with a loud noise, crushing vast numbers beneath their weight- while from the temple of Minerva there went up the war-cry and the shout of victory.
All these things together struck terror into the barbarians, who forthwith turned and fled. The Delphians, seeing this, came down from their hiding-places, and smote them with a great slaughter, from which such as escaped fled straight into Boeotia. These men, on their return, declared (as I am told) that besides the marvels mentioned above, they witnessed also other supernatural sights. Two armed warriors, they said, of a stature more than human, pursued after their flying ranks, pressing them close and slaying them.
These men, the Delphians maintain, were two Heroes belonging to the place- by name Phylacus and Autonous- each of whom has a sacred precinct near the temple; one, that of Phylacus, hard by the road which runs above the temple of Pronaia; the other, that of Autonous, near the Castalian spring, at the foot of the peak called Hyampeia. The blocks of stone which fell from Parnassus might still be seen in my day; they lay in the precinct of Pronaia, where they stopped, after rolling through the host of the barbarians. Thus was this body of men forced to retire from the temple.
Herodotus does express the narrative voice that does believe in the miracles surrounding the temple at Delphi but he also tells us that he is relying upon sources who claimed to be eyewitnesses and that what he believes is what the Delphians maintain. So he allows the reader to make up his and her own mind about what he believes and why (e.g the blocks of stone can be seen to this very day!)
That's the way a historian writes according to all passable conventions of historical writing: despite the bias of the historian the historian writes at arms length and allows the reader to understand the reason for a bias or belief, and is thus free to make up their own mind.
In fiction (and the gospels and Genesis and Exodus) the reader is simply told that as surely as the coach turned into a pumpkin at midnight so various other miracles really did happen.
It is also relevant to note that Herodotus is writing a work that some scholars have compared with the Primary History of the Old Testament. They argue that he is writing to demonstrate the power and will of Apollo in history, and that his temple at Delphi is a counterpart to the Jewish Temple at Jerusalem. That is, he is writing "theological 'history'", not "real" history anyway.
