I didn't realize there was a debate about what Alfred said to Harry. Thank you for an interesting early morning read.
I assumed Alfred meant "Once upon a time, the Warden of this island was the only one, (and although other wizards and beings may have helped stick inmates in here), it was solely the Warden's job to see that they stayed in and other people stayed out." Parenthetical statement is purely my inference.
Some other inferences to be made, some are obvious, some are my own, some are in a spirit of playful ridiculousness:
1. The island hasn't had a Warden for a hell of a long time, BUT with wizard lifespans being what they are, it could simply be a "we will get to it eventually" thing.
2. The job (or possibly Mantle) of being the Warden has very specific duties...and at least one perk: You get to be a Bond-villainesque shadowy character with his own island. I have a hard time believing that wouldn't appeal to Harry somewhat.
3. There will be eventually a Braveheart reference with Harry saying "I told you, its MY island."
4. It is easy to see how the Council could extend the Warden idea to the Warden(s) a la kazimmoinuddin's post. Perhaps the Warden of DR had his own SWAT team to hunt down and imprison certain beings. (Let's all try to imagine how, if skinwalkers are in minimum security, how hardcore someone was to imprison the things in the lower levels.)
5. I like the idea of an "original cloak and sword", something Nuccio's swords were based off of perhaps. Maybe Harry finds them. Let's hope it's a rapier or some other type of fencing steel, Harry doesn't seem the broadsword type.
6. Perhaps, if the poster with the multiple Warden of DR theory is correct, Alfred is referring to the British-inflected voice guy Harry found in a crystal.