To me the myth is actually that Bettas do not need/cannot handle any fiber. Feeding pea's is not harmful when offered even on a weekly basis. Just because an animal is carnivorous doesn't mean they do not consume any fiber!
A blanched skinless pea is a great source of fiber. Most importantly, because they are
carnivorous it is not fully digestible. This flushes out the Bettas digestive tract, often times relieving constipation or removing any blockage.
Have you guys looked what is in Betta pellets? It isn't just protein. In fact, one of best foods on the market (New Life Spectrum Betta Formula) has algae meal, that consists of seaweed, kelp and Haematococcus pluvialis, a premium grade of natural spirulina, as well as a fruit and vegetable extract that consists of spinach, red and green cabbage, peas
, broccoli, red pepper, zucchini, tomato, kiwi, apricot, pear, mango, apple, papaya and peach.