Potatoes have a GI of about 70, comparable to white rice or white bread (pure glucose is 100). So pretty bad.

The “potato diet” will only lead to weight loss if you’re eating fewer calories than you’re expending, same as anything else. You could eat nothing but bacon grease and you’d lose weight if you were eating below maintenance.

The effect of food on blood glucose if a matter of how easily it is converted to glucose. Starches like those in potatoes, flour, rice etc are basically converted nearly as easily as straight sugar.

I think there might be other things going on with the potato diet. In Slime Mold Time Mold's study participants seemed to lose more weight when cooking potatoes with the skin on. My wife and I recently tried three diet for about a week with good weightloss. However, my wife who prefers potatoes cooked with skin quickly got abdominal pain and dropped the diet. When I started to eat all the potatoes I had cooked for her, I got similar issues. I think there is a good chance that the solanine which is mostly in the skins messes with people's digestive system and contributing to weightloss. Poisoning yourself doesn't strike me as a valid approach to weightloss. Supposedly someone tried supplementing solanine and didn't see the weightloss. I don't trust that though, since there was a more comprehensive trial or a believable alternative explanation