That's related to what FODMAP are: fermentable carbohydrates. They'll be decomposed in the fermentation process for sourdough bread.
It has been noticed by many people who think they're gluten intolerant that they digest sourdough bread much better. The likelyhood that this is linked to the changes incurred by the fermentation process is high, since, as you said, that's the only difference.
It's more the opposite: FODMAPs are non-fermentable carbohydrates. If they were fermentable by yeast, they would be fermented and you wouldn't be eating them. The whole point is that yeast doesn't metabolize them but your natural intestinal flora does.
Sourdough is a mixture of yeasts, lactobacter, and acidophilus in various proportions. The latter two will metabolize the FODMAPS, either while proving the bread or in your gut. One of those choices can cause discomfort and the other a delicious food product.
It has been noticed by many people who think they're gluten intolerant that they digest sourdough bread much better. The likelyhood that this is linked to the changes incurred by the fermentation process is high, since, as you said, that's the only difference.