My understanding is that the liver prioritises dealing with the alcohol before releasing the glucose, as the alcohol is a toxin which needs getting rid of.
This therefore, slows the release of glucose into the blood, so for type 2 on diet only or non-insulin type meds it will reduce the spikes of 'carbs' eaten - I know I can have more carbs if I have a glass of wine with the meal.
Re T1 or T2 on insulin type meds, the alcohol messes up the calculations of insulin required, leading to hypo territory.
One of the more scientific bods will be around soon but I think that's the jist of it.
Mary x