I think something like "brain in a vat" is the best inference from observing magic. [EDITED to add: of course I mean after getting very good evidence against deception, insanity, etc.]

More precisely: if you find evidence that something deeply embedded in the universe is best understood at something like the level of human concepts -- it matters what words you say, whether you really hate someone else as you say them, etc. -- then you should assign more probability to the hypothesis that the-universe-as-it-now-is was made, or at least heavily influenced, by someone or something with a mind (or minds) somewhat like ours. That could be a god, a graduate student in another universe with a big computer, superintelligent aliens or AIs who've messed with the fabric of reality in our part of the world, or any number of other things.

In a manner of speaking this is obviously correct for the Potterverse (either Rowling's or Yudkowsky's): in that universe, magic works; and indeed that universe was designed by an intelligent being or beings, namely Rowling or Rowling+Yudkowsky. It probably doesn't work "internally" for the original Potterverse -- I've no idea whether Rowling has any particular position on whether within the stories the world should be thought of as created by intelligent beings -- but I'm guessing that it does for Eliezer's.