<Credits: Renson Seow>
No need to kancheong. Straight distance Singapore to Batam is about 29km. Most modern guided antitank missiles that are man portable can do 5km thereabouts at best. Syrian makeshift lpg container launcher ("Hell Cannon") can do 1.5km if the gunner wears his lucky underwear.
Ballistic rockets can reach further, eg a katyusha rocket can touch 30km, but you need a truck to mount it on. Haven't heard of any man portable ballistic rockets.
So in other words, these wannabe terrorists were a bit optimistic about their weapon's range.
Qassam 3 on a good day reaches 16km. Plus it doesn't have a guidance system. They can't fire a ranging shot and correct with an observer, so they'll have to rely on math - calculating the landing of the round based on their knowledge of how much propellant will send the rocket how far, at what angle...
And that's assuming the rocket has consistent quality control so that it flies according to the calculated trajectory. Add a rocking boat to that and you have a devil of a time trying to land your rocket at that distance.
So yeah, I think things will be alright. We're pretty safe from the rocket method.