"These 1 billion users may also be encountering Gemini in all those places, but that’s not the MAU metric. Pichai is talking only about people who are opening the Gemini app or visiting the Gemini web interface to enter a prompt or access Gemini Live. If you used Gemini only once in the past month, you are part of this cohort"
AI Overviews are not accounted for.
So as usual with these type of consumer choice/preference claims, it's sensible to be cynical. At least the Pepsi Challenge couldn't secretly swap out your water tap at home for a soda fountain, record you drinking it and claiming you prefer it to Coke. https://en.wikipedia.org/wiki/Pepsi_Challenge
Though I guess it's possible all their 1B users are concentrated in India and China and I'd never know.