Every PR who does his NS is a foreigner. Nothing new there. You will NOT automatically have your citizenship canceled UNTIL you serve NS. This doesn't matter what the law says (the one you are quoting) as MINDEF rules overrides all others. Believe what you want, okay?
If you get something that verified your loss of Citizenship prior to your entry into NS then you are free and clear. However, if you don't and you don't do your NS because you assumed wrongly, you will be a deserter and all that I mentioned before will pertain.