It implies that he valued that god above any others that he had leaned of or imagined to ever happen, I suppose.
It is not a statement of existence, but of his personal dedication - unless he presumed to be some sort of validator for the existence of gods, I suppose. Which would be odd.
But another, perhaps more likely possibility is that a historical Melchizedek never existed and his story was always allegorical or inspirational, in order to emphasize the god of Judaism.