Actually some of the cashiers have no idea what they are doing. You should try, nothing to lose anyway. Once she scans your QR code and if it shows RM 5 discount then you pay, else you don't pay.

Honestly, I still think it's a nationwide promo and shouldn't be restricted to Peninsular only?
In a way yes, that's how I paid my friends in China. Either with WeChat Pay or Alipay.