That's correct; prelaunched fleets are for all intents and purposes already "launched" - if your target drops below the bash limit, goes into vacation mode, or is exiled to c200+, your fleet
will still launch, and the ships in that fleet
will not engage in battle at your home fleet.
In reply to making it a fake attack; consider it from the other side. You have a person who has scanned and calced an attack, set aside their ships and launched a fleet (which could be up to ETA 1), and then for reasons totally out of their control, their fleet is recalled. They've wasted a slot, they've wasted an attacking fleet, scans, calcs and time with a target that they
should have been able to attack, only to be denied. For most people, that means that they wont be getting roids that day, which is generally how this game works.
Sure, it might be better if those later waves recalled. after all, that's fewer roids and the target has already lost their fleet or whatnot. But i'm just making the case that its not so clear cut. Attackers arent always bad guys - you (should be :P) an attacker to people as well.