The cases are being under-reported too. The question is whether deaths are being under-reported to a greater degree than cases. Even the test-positivity rates vary from state to state not only because of the extent of infection in different states but also because some states contact-trace and test while others don't. Its a mess. And to see people try to fit models to this data and fight amongst each other is darkly hilarious.
Plus, at least in the first wave in Bangalore (where I grew up), there were no instances of crematoriums melting down and such. So the under-reporting of deaths was likely not as bad in the first wave as it is right now, but the under-reporting of cases was certainly quite large as testing capacity took a while to come online.