iirc, from older articles, which differ from this nice result, bile acids contain cholesterol(s) and they're generally reabsorbed in the intestines, so the fiber is conjectured to bind with some before reabsorption, expelling the bound fraction of circulating cholesterol in feces.

this result in the paper is very interesting in the conjecture is that the gut microbiome is altered in a beneficial way, and that the effect (with the resulting lowering of cholesterol) persists for weeks after even 2 days of oats.