"In at least one case cited in the investigation, baking powder was mixed into food to make tourists physically unwell."
The only ill effect I can find from overconsumption is a "tingly sensation on the tongue". Of course, that doesn't mean the 'poisoner' wasn't ignorant of this, and genuinely did it trying to make them sick. Or maybe they simply said, "If you feel your tongue tingling, YOU ARE DYING!!!".
I feel like "make physically unwell" here just means they'll taste something gross, not realize it is the baking powder, and treat the feeling as if something is wrong with their body. I know mixing up baking soda and baking powder has made for some pretty unpleasant tasting food for me.
Could it be a symptom of high altitude oxygen deprivation?
Baking powder is used by some athletes as it is shown to enhance performance.[0] I find it more realistic that they are adding it for that effect than making anyone unwell. As its not really something that makes you ill.
It’s a leavener when you get it wet, so swallowing enough will definitely feel like digestive upset from all the gas.