What GB refers to is more something happening in the writer's mind. By saying or assuming that sufficiently explained magic is science, it creates a kind of corset the writer has to fit his writing in.
Compare, for example, the treatment of magic in Lord of the Rings with the treatment of magic in Harry Potter. In LotR, magic is largely unexplained, and even largely invisible (Tom Bombadil notwithstanding). There seem to be a few rules, but we never really find out what they are; Magic is something that is present in the setting, but never really explained.
In HP, on the other hand, magic is a developed topic of study; sure there are some mysteries, but most of it has been formalized to such a degree that you can develop a curriculum to teach its proper use to teenagers.
Now, both approaches can be used quite well, both definitely have their place, but that does not mean they are universal rules. The "magic as science" approach makes a certain amount of sense when you tell a story from the POV of a traditional Wizard-type character who derives his power from studying the laws of magic; but if your story is told from the POV of, say, a bunch of hobbits who haven't got a clue about magic and who will never use it in the story, it's better to leave your mind open and not bother with creating an elaborate ruleset, as this allows you to be more imaginative in your applications of magic.