Why does Harry think that Dumbledore has never been in the Room of Requirement?

Question

In Chapter Thirty-One of Deathly Hallows Harry has the following thought about the Room of Requirement:

Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle. Of course, Dumbledore and Flitwick, those model pupils, had never set foot in that particular place, but he, Harry, had strayed off the beaten track in his time at school — here at last was a secret he and Voldemort knew, that Dumbledore had never discovered —

But Harry was aware that Dumbledore had indeed set foot in the Room of Requirement. In Chapter Eighteen of Order of the Phoenix he says the following to Dobby:

“— and if you really needed a bathroom,” said Harry, suddenly remembering something Dumbledore had said at the Yule Ball the previous Christmas, “would it fill itself with chamber pots?”

This is a reference to Chapter Twenty-Three of Goblet of Fire:

“Oh I would never dream of assuming I know all Hogwarts’ secrets, Igor,” said Dumbledore amicably. “Only this morning, for instance, I took a wrong turning on the way to the bathroom and found myself in a beautifully proportioned room I have never seen before, containing a really rather magnificent collection of chamber pots. When I went back to investigate more closely, I discovered that the room had vanished. But I must keep an eye out for it. Possibly it is only accessible at five-thirty in the morning. Or it may only appear at the quarter moon — or when the seeker has an exceptionally full bladder.”

Harry clearly took Dumbledore’s comment seriously, as (back in Order of the Phoenix) when he told Ron and Hermione about the room he invoked Dumbledore:

“This room isn’t just some mad idea of Dobby’s; Dumbledore knows about it too, he mentioned it to me at the Yule Ball.” Hermione’s expression cleared.

Dumbledore told you about it?”

Just in passing,” said Harry, shrugging.“

Why, then, in Deathly Hallows does Harry think that Dumbledore has never been in the Room of Requirement?

Answer 1

I think we should parse "that particular place" as the version of the Room which is full of random illicit stuff. The Room is presented as being able to show different versions of itself, and it can swap between them so long as it is unoccupied. (In Deathly Hallows this is a minor plot point as it can't be used as a safe zone at the same time as allowing access to the illicit-objects collection.) From what Harry knows, Dumbledore has seen the Room when it presented an assortment of chamber pots, but on the basis of that overheard conversation it would appear that Dumbledore hasn't penetrated the true nature of the Room. (Subsequent stories have shown him using it; that's still consistent with him not spilling every bean in his chat with Karkaroff, where he was mainly trying to be cryptically unhelpful.)

At this point, Harry believes that the diadem is hidden in the illicit-objects version of the Room, which is a single configuration that has been used by many students at different times. In accordance with the general working rules of the Room, it would be accessible to people who are trying to hide or retrieve a forbidden object. That matches all the known uses of that version of the Room, including the Half-Blood Prince's book, Malfoy's teleportation cabinet, and Trelawney's stash of sherry. If Harry, or anyone else, used the Room with a different need in mind, then they wouldn't have access to those objects.

This explains Harry's thoughts that Dumbledore and Flitwick were too goody-goody to have been in "that particular place": they wouldn't have had anything so naughty that they had a great need to hide it. Dumbledore (in Harry's view) has encountered the Room in a different form, and doesn't know that it also holds a stash of contraband.

Answer 2

You make two wrong assumption here.

First one is that Harry is not explaining his thoughts but Riddle's. It's Riddle that would be absolutely certain that Dumbledore would never found the Room of requirements, (and more importantly, the specific room for hiding things).

Second one is the timeline, and is in fact a proof of the first one. If you believe Dumbledore (which you should not), he discovered the room, by accident, the day of the Yule Ball, decades after Riddle decision to hide the diadem there. So Tom clearly was right about it.