Remember Delta wasn't even around until late summer. Biden's speech was barely an exaggeration at the time when we were dealing with the original strain. Was it 100% effective at preventing all infection entirely? No. It was 100% effective in preventing severe symptoms, hospitalization, and death, and was 95% effective in preventing symptoms entirely. 95% reduction in infection and 100% reduction in death is close enough to round up for a non-technical speech.