They showed that stool from RYGB mice caused some amount of weight loss that stool from non-RYGB mice didn’t. They speculate that it is due to bacteria change, which seems well-founded. But it doesn’t seem like they were able to tie bacteria levels to weight loss, which is a pretty big missing puzzle piece.
I also do not have access to the actual article but read it to mean that surgery leads to a fairly rapid change in microbiota composition before weight loss and also not the result of calorie reduction: the surgery causes the change in the microbiota, not the weight loss nor calorie reduction resulting from the surgery. The surgery, by whatever mechanism, then leads to weight loss. That is solidly established. What this adds is some evidence that at least part of the mechanism by which the surgery does that is by way of that change in microbiota composition, since transplanting that microbiota leads to rapid weight loss as well. Although from this summary of the study the weight loss of the sham/transplant group was 20% of the group that had the surgery. So it seems that the bacteria are not the only factor. It also is not clear how long the groups were followed: without surgery was the changed microbiota sustained? Or did it and then weight both revert?