Cooking Russet Potatoes in the Instant Pot hydrates the starch granules way more than simple boiling. The resulting potatoes don't need much to turn them into mashed potatoes. Really, the additions are more for flavoring than thinning and smoothing. This recipe from Salty Marshmellow is the general method, though the sour cream is never added and usually not that much butter, either. Because the cooking liquid is driven into the potatoes, seasoning the cooking water can give it a boost.