"We reclassified non-drinkers as former drinkers if they had any record of drinking or a history of alcohol abuse in their entire clinical record entered on CPRD before study entry."
This will not capture a significant percentage of the former drinkers who are non drinkers. So contrary to your assertion, the current study did not truly separate out the two categories.
However, their defined data (raw data) contains former drinkers as well. It is just that the way their analysis (reclassification) for the result merges former drinkers with non-drinkers to make it easier. It is similar to given 1, 2, 3, 4, and 5, I want to reclassify it to 1, 3, and 5 to make it more distinctive.
We used the most recent record of alcohol consumption in the five years before entry into the study to classify participants’ drinking behaviour. In light of current debates on the U or J shaped relation observed between consumption and aggregated cardiovascular disease outcomes we defined five categories of drinking: non-drinkers (Read codes such as "teetotaller" and "non-drinker"), former drinkers (those with codes for "stopped drinking alcohol" and/or "ex-drinker"), occasional drinkers (those with codes for "drinks rarely" and/or "drinks occasionally"), current moderate drinkers (codes such as "alcohol intake within recommended sensible limits" and "light drinker"), and heavy drinkers (codes including "alcohol intake above recommended sensible drinking limits" and "hazardous alcohol use").
This reclassification does not invalidate their study or make it look as bad as your impression is. Also, if you really look at the trend in their figures, you should see that it is OK to merge former drinkers with non-drinkers. This study shows a proof of concept and could be refined for further studies.