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Consider the first human who was not lactose intolerant.

You might consider this. Lactose intolerance and the breastfed baby

Lactose is the sugar in all mammalian milks. It is produced in the breast and is independent of the mother's consumption of lactose. It is present in a constant concentration in breastmilk.

Primary (or true) lactose intolerance is an extremely rare genetic condition and is incompatible with normal life without medical intervention. A truly lactose intolerant baby would fail to thrive from birth (ie not even start to gain weight), and show obvious symptoms of malabsorption and dehydration - a medical emergency case needing a special diet from soon after birth.

Anything that damages the gut lining, even subtly, can cause secondary lactose intolerance. The enzyme lactase is produced in the very tips of folds of the intestine, and anything that causes damage to the gut may wipe off these tips and reduce the enzyme production, for example.

gastroenteritis

food intolerance or allergy (In breastfed babies, this can come from food proteins, such as cow's milk, soy or egg, in the mother's milk originating in her diet, as well as from food the baby has eaten.)

coeliac disease (intolerance to the gluten in wheat products)

following bowel surgery

In adult lactose intolerance, there is an age-related decline in the production of lactase-phlorizin hydrolase, the enzyme which cleaves lactose. The persistence of the enzyme past infancy is the mutation this article speaks to. (In particular, the mutation is found in a control region separate from the gene itself.)

And yes, you are correct that there are other conditions, e.g. intestinal inflammation, etc. which can reduce the presence of this enzyme, causing temporary or permanent lactose intolerance. But the point is that the normal presence of the enzyme into adulthood is an adaptive mutation. Diseases or conditions which can strip this advantage are interesting but beside the point.