The chapter emphasizes that it's a separate personality system that's running Harry at that point (which doesn't prove it's Voldemort, but is suggestive). E.g.:

that's not Harry--

You know. About his dark side.

Although it's not absolutely definitive; Dumbledore's line in reply is

But this is beyond even that.

which argues for "he's damaged" as you suggest rather than "he's alien [and Voldemort]" as I'm suggesting.