The thing about cooking is that you can put whatever you want into your recipe.
“Authentic” recipes never existed- every grandmother out there uses a slightly different recipe to each other.
Not disputing authenticity of the Kolkata biryani.
It’s just that putting potato in a biryani goes against what the biryani is in rest of India.
It’s been something that’s evolved out of necessity in Kolkata. Potato was added because the cost of meat was prohibitive in those days and was continued.
I think it does add a bit of starchyness but also often adds quite a bit of sweetness too which to me is undesirable.
Do like a variant of the Kolkata style biryani which I make sometimes. It’s like a hybrid style between both Hyderabadi style and Kolkata style.
Basically four major styles of biryani in India (not including Karachi biryani as that’s a Pakistani dish now).
Bengali , Hyderabadi , Lucknow and Kozhikode(Kerala). All have similarities and yet some very important differences