If you were to check to
Singapore website for Malaysians to be accepted for the ASEAN scholarship, you will find that they are meant for Sec1, Sec3 and A-levels. There is also a scholarship for University degree but the student need to achieve good results at A-levels. So, in this case, the student with only SPM results will not be "a shoo-in for that scholarship" if you maintain the context of being admitted to NUS.
O, by the way, I don't think Singapore will accept any student based only on their fantastic paper results as you suggested here.
FYI, my nephew got the chance to apply for an ASEAN scholarship for A-levels in Singapore. The number of A's that he got was not really enough. He was actually accepted based on an essay he wrote (in English). Let me tell you this, with the glut of A-results, especially from Malaysians, it is not enough to get in on a string of A's. The student need to impress in their maturity of thought, lateral thinking and possibly being able to think out of the box. So, A's aren't enough and certainly not a shoo-in. Basically Singapore are not interested in paper-chasers who depend on rote-learning and memory work. Critical thinking is the key. Without that, don't rate your chances of getting that prestigious scholarship.