That can't be right. It is the pink IC and pink IC alone.

Neil Humphreys would fit the criteria in first comment just nice without pink IC. But he needs his British passport to go back to the UK to see his parents, so he is not Singaporean.

Ben Davis was signed up as a Singaporean by his father (passport and all), he had no voice as to when he would return after deferment to serve National Service. Yet he would be more Singaporean than Neil Humphreys as he would have fulfilled all of the above first comment criteria plus pink IC.
On the other hand, pink ICs having immediate male relatives who have been enlisted for NS should not be a criteria for being Singaporean, because of it being by accident of birth.

No hard and fast rule. NS doesn't mean Singaporean even though "everyone" does it.