I know: that's what I meant by saying that perhaps JKR didn't mean to imply in that letter that they were already under the Fidelius charm, since that makes timing work much better on that (it was later to come, perhaps mid October), but it's hard to think what else might keep him literally stuck there. Perhaps Albus had already got hold of the Invisibility Cloak and he was persuaded that he couldn't go out without it?