While this is true, it's not necessarily the best solution. You could put the lid on the pot or cut a piece of parchment paper and rest it on the surface of the liquid to reduce evaporation or both.
Every time I see oil called for in cooking beans, it's about controlling foaming for pressure cookers. So the foam doesn't clog the vent. Never seen it for beans otherwise.
As for controlling evaporation I think there's too many variables to make a sound argument based on what you told us. Things like the surface area of the pot the amount of oil how much agitation from boiling and so on.
Agree, have only heard of adding oil to control foam.
IMO, if you need to control evaporation, perhaps the pot you are using is just not big enough for the amount of water you need for the beans you are boiling.
Then again, which is normally done. Boil beans rapidly? or simmer them slowly.