Requiring a Google account would indeed not fly. The project was called next billion users and it's objective was to seek internet penetration into areas of India where internet literacy is nonexistent.

As a part of that initiative you had manufacturers building super low-cost Android smartphones that came preloaded with Google apps and people could put a SIM card in. The backing system of GPay in India called UPI works similarly so this app could never have that second layer of auth as a mandatory wrapper and spread where Google wanted it to spread.

Those decisions gave them 200M users on the app in the country.