Rhubarb is on sweet side all over the word when Sri Lankans cook it to a spicy and aromatic curry. Its not a common vegetable in most of the parts In Sri Lanka as it grows in cooler climate in the Hilly parts of Sri Lanka. My mother in law is a good cook. She used to cook this all the time with potatoes. If you cook it by it self its not giving this thick starchiness to the curry. You can always adjust the sourness by adding a bit of salt and sugar.
Serve this with plain rice to get the real taste. If you are a vegetarian or a vegan omit the Maldive fish flakes. I add Maldive fish in my curries as I learnt with Maldive fish flakes in almost all the vegetable curries. Maldive fish flakes are made of Dried and smoked Tuna. It gives a different flavour to the curry.