Theoretically could. But it didn't happen, and nobody even made a PoC or demonstration. And full source code was released within three weeks of the initial Jail

Theoretically could. But it didn't happen, and nobody even made a PoC or demonstration. And full source code was released within three weeks of the initial JailbreakMe release, so it's not like it was made particularly hard if someone wanted. (I don't count that thing you linked; it only affected jailbroken devices who installed a specific package and didn't change their password.)