Harry expresses a lot more skepticism than that a lot more recently. Chapter 86:

"So I see," Harry said. His hand rose up, rubbed the scar on his forehead. "But... okay, if this is really all we know... look, I'll just put it bluntly. How do you know that the Dark Lord actually survived?"

And:

The scarred man turned back to face the Headmaster, though his wand stayed pointed low and in Harry's general direction. When he spoke his voice was gruff and businesslike. "I have a lead on a recent host of Voldie's. You're certain his shade is in Hogwarts now?"

"Not certain -" Dumbledore began.

"Say what? " Harry interrupted. After having nearly concluded that the Dark Lord didn't exist, it was a shock to hear it being discussed that matter-of-factly.