The revelation of the presence of these young terrorists from the three countries is to give notice to the countries that they better take the threat seriously. And these countries have no choice but to bolster up their defensive mechanism and organisations to prepare and counter the threats. Manpower, equipment and time would have to be allocated to prepare for the inevitable, and as they said, it is a matter of when. So, a simple act of flashing the video in cyberspace would have the effect of making countries tying down their resources and manpower for something in the dark that may or may not happen and without any idea when or where they will hit. The IS is having the upper hand, they have the initiative, when and where to strike.
On the international scale, such wars of words would put big countries like the USA and its allies, participants in the Coalition of the unWilling shitting in their pants. All IS needs to do is to post a few threats in the net and their anti terrorist organizations would be kept very busy shadow fighting. And it would be worse when the threat is more specific, with time and place revealed.
This psycho warfare could drive many countries crazy, wasting time, resources and manpower for apparently nothing. The cost to IS and other terrorist organizations is practically negligible, but not to the countries they are targeting. No country can ignore the threats and Singapore is deploying the Army boys and the Home Teams and NSmen and whatever available to be ready, and cannot afford to be caught with the pants down. Then everyone will be pointing the finger, they told you. Why didn’t you do something?