Baking powder releases carbon dioxide. Baking soda just creates aqueous bicarb ions and a more basic solution (which is the key to a faster reaction).
Heating a solution of aqueous bicarbonate will release carbon dioxide, too. But since we have delicious onions and stuff in there too, let’s walk through my thought process: Baking powder is baking soda + weak acid + cornstarch (to prevent premature reaction). Since the speed-up for the Maillard reaction works by deprotonating amino groups to make them more nucleophilic, the acid-base reaction that releases CO2 when using baking powder will still occur with just baking soda + food (ie: the protiens in the food are acting as the acid). You’re probably right that using baking powder would produce more CO2, or at least produce it faster, but reducing carcinogenic side products for Maillard reactions via CO2 is a low-priority concern for me anyway. Just a fun curiousity that occured to me when reading the wiki page!
Sorry if my carbon dioxide subscripts don’t work. I don’t think my client supports all the fancy markdown, but i tried my best.