This is pretty well understood and documented.

https://www.nhs.uk/conditions/lactose-intolerance/causes/

"Bacteria in the colon break down the lactose, producing fatty acids and gases like carbon dioxide, hydrogen and methane.

The breakdown of the lactose in the colon, and the resulting acids and gases that are produced, cause the symptoms of lactose intolerance, such as flatulence and bloating."

The tricky part is that there are multiple species of bacteria able to digest (ferment) lactose, and the composition of your microbiota is highly variable at the individual level and also over time.

In practice, your reaction to lactose is difficult to predict, regardless of your production of lactase.

That’s the non controversial part. The bit that seems off is that there is an immune response.

Right. I guess I had misunderstood the comment as implying that lactose intolerance was not a result of insufficient lactase production.