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This article is odd because he first says

and then
He seems to forget the definition he gave and reverts to the popular but wrong idea that being tipsy is venial and moderately drunk mortal.

He didn't forget anything, but you're operating on a false dichotomy that if something isn't mortal sin, then it's not sinful at all.

So, the principle is quite clear, any degree of "tipsiness" must be justified by some kind of reason that's proportional with the degree of "tipsiness" (assuming other factors don't apply, such as I articulated above, if you're an angry drunk or prone to impurity when drink, etc.).  Consequently, a small amount of tipsiness can be justified simply be a need or desire to relax or to recreate.  As one becomes MORE tipsy, there needs to be a proportionate reason, i.e. is that next level of tipsiness justified by giving you somehow a better and more thorough relaxation or relief from anxiety.  If it isn't, i.e. where the relief you get or the recreation you get isn't increased by that degree of tipsiness, the degree of tipsiness is disporportinoate to the justifying benefit and therefore becomes venial sin, very analogous to gluttony.  Yes, a certain amount of simple pleasure in eating food justifies eating delicious food, and one needn't be always consuming nutrition only without any regard for the pleasure and satisfaction of eating (we know that eating has many benefits, increasing serotonin in the brain, etc.)  BUT as soon as one star going for increased degrees of pleasure that have gotten out of proportion to those benefits of eating, it becomes venially sinful.  Very similiar principles apply to alcohol, the difference being is that (if I recall) St. Thomas says that it's nearly impossible for gluttony to become mortal, and the reason alcohol can become mortal (vs. just venial) would be on account of that loss of reason.  Now, even loss of reason can be justified with a proportionately serious reason, such as, say, someone needs to perform some surgery on you and you lack anaesthetic, so you drink enought to lose consciousness.  While there's a total loss of reason, the circuмstances can justify it.