I'm not great with numbers, but...
6.8 mmol/l from the meter in the drink times 18 gives 122 mg/dl
122 mg/dl is the same as 1.2 grams per litre.
That's low, practically nil.
Whether it meets statutory definitions of zero carb, I have no idea, but given that meters are designed to measure blood, not drinks, there's probably a whole lot of things going which means the meter does not accurately measure what's in the drink, there will no doubt be specific equipment for that.
Still, I do occasionally meter a diet coke if it's come from a gun and I'm not convinced the bar staff have hit the right button, and those always show LO.
Out of interest, what was the drink?