Peat does talk about the beneficial anti-inflammatory effects of lactobaccilus, separate from its lactic acid production. However, lactic acidosis isn’t a problem of lactobacillus presence, but rather an imbalanced ratio of lactobacillus to lactate-utilizing bacteria like anaerostripes, which convert lactic acid into the useful butyrate.
When the bacteria reaches the colon - yes, it is usually beneficial. The issue the study discusses is that in people with slow digestion (basically majority of people) this bacteria colonizes the small intestine (SIBO) and leads to all kinds of issues. So, for probiotics to work they should be developed in a way that do not release their "payload" until they reach the colon.