The loophole wasn't actually kept secret at all, because there's no actual loophole.
Someone just happened to get a Kindle that slipped a QA step and knew what do to with that fact
. And we got extremely lucky with the way that QA image was packaged (which may, in fact, have been a quick shortcut to make the QA process more streamlined at the time, e.g., release crunch).
It's entirely plausible that this happened to get fixed just because someone at lab126 was in the vicinity of the updater's code for whatever reason, looked at it and went: "well, that's dumb".