I think you're looking at it the wrong way. If the writer paints themselves into a corner where they need a deus ex machina in order to continue the story, then they've done a poor job of it, and need to back up and reconsider where the story was going. The argument isn't "Harry needed to be rescued in an unbelievably fantastical manner because otherwise he would have died and the story would have ended", the argument is "the story shouldn't have taken Harry to a crisis where he couldn't have gotten out of it in an in-universe-believable way".

(FWIW, I don't really object to this particular part in the story; I think a flying car is pretty consistent with the Harry Potter universe. And the poster upthread must have misremembered the scene; his friends merely came to "rescue" him from being locked in his room. He wasn't hanging out a window and wouldn't have died.)