Since Dumbledore didn't give anyone else access to an undisturbed scene, it's merely the best conclusion that anyone could draw who wasn't an actual professional investigator / smart person.

Actual professional investigators would know not to come to any conclusion without actually investigating the scene. (Whether or not they trusted Dumbledore to investigate is a different matter.)

Smart people would think:

They have already marked the Night of Godric's Hollow, as reported by Albus Dumbledore, as an anomalous and potentially important event. They have wondered why it happened, if it did happen; or if not, why Dumbledore is lying.