It is true but on a case to case basis. It is a technical loophole with limitation. It is technically true when you have a 30 days permit, you went KL for a few days in your 15th day stay, your passport will be reset since you have already left Singapore. So when you come back you are entering Singapore and get a fresh 30 (or 15) -days (case-to-case).
But if you leave Singapore on your 28th or 29th day and go to JB, there is a higher chance that you may not be granted a long stay when you come back. Since its a case to case, the number of days left will be a factor, and/or the destination you go to. The age and relationship may also be considered, i.e. grandma or baby. Or your reason, health or medical treatment, etc etc.