It's not clear to me that the bugs have FTL indeed we don't see them use such in movie or book. Moreover sending a single small rock makes zero sense FTL and knowledge of humanities only major world would have allowed them to wipe out the threat in a stroke.
You could A) warp in a rock of sufficient velocity that a small one destroys life on the planet or B) warp in and move a bigger rock in system.
It only makes sense as a false flag by the humans.
You're ignoring genre conventions. Every soft scifi movie is nonsense if you look at it from the perspective of real-life physics. Sending a single rock via unspecified FTL to attack Earth makes as much sense as the human-piloted fighter spacecraft in Star Wars. The aliens are capable of bombarding Earth across interstellar space because it makes for a good visual spectacle. Watch (or more likely read) hard scifi instead if you need everything to make logical sense.
You could A) warp in a rock of sufficient velocity that a small one destroys life on the planet or B) warp in and move a bigger rock in system.
It only makes sense as a false flag by the humans.