"LAUNCHING MISSILES (click here very quickly to undo)..."

"cancel" and "undo" is not the same thing.

You can cancel the launch. But you can't undo it. It is thermodynamically impossible.

Right. But that's how Gmail is marketing it. "Undo send" gives you the impression that Gmail is reaching through the tubes and yanking your message back for you, but in reality it's just cancelling a launch that didn't happen yet.

Yes, but there's a compensating transaction you can apply to the missile up until the time it reaches the target.

You're right that it's not a "real" undo, but it is better than a warning with timeout; on a warning, it demands that you click (habitual action) after which there's no going back, but with this system it demands nothing, the message stays up, and you click (non-habitual action) only when you want to cancel.

It is a warning where instead of the word "cancel" they use "undo".

Except it's not the same: on a warning, you click OK and then A) the warning is gone and B) you can't get it back.