what i also wonder is what happens to the Legder sticks that don't go through this update - can they continue to be used without problems?
Probably. But you can't ever be sure that this backdoor hasn't been there all along, as pointed out by others upthread.
Yes, I don't see the firmware creating the backdoor. The fact that the new firmware has already circulated, even briefly, can now be analyzed by hackers to be able to discover that backdoor.
Allegedly, and the fact that they have made it very clear everywhere, the Nano S model will be the only one that will not have that back door.
But how now are we to know? Doubts were left in the air, there is not much way to remedy it.