This recipe has more baking powder than most vs ingredients. Only one person said it was bitter. Does the type or brand of bp matter?

Any brand of baking powder should work! 1 to 1 1/4 teaspoons of baking powder per 1 cup of flour is a general rule; in this case, that amount is slightly higher (by 1/2 teaspoon) to balance the denseness of the pumpkin puree.