It is not urban legend. Not sure where you heard that. It is now a requirement to exclude obese stool donors and to tell stool recipients during informed consent that they may gain weight as a result of the transplant.
Stool samples transplants of course should be taken only from donors with no health issues. People who are obese have an obvious health issue. A form meant to protect the drug companies does not prove anything. They probably mention “bleeding eyeballs” and other nasty sounding side effects as well. As I said in a previous post if someone is suffering from digestive issues where they have diarrhea and are constantly pooping out almost everything they eat before their body can absorb it... there is a good chance they will gain weight when their problem goes away if they do not adjust their consumption level.