Most vaccines are less than 100% effective at preventing infections. Vaccinate 10,000 people and intentionally expose them to the thing vaccinated against and you will get some infections.

When there are significant numbers of infected people in the population, adding other measures to the protection provided by vaccines decreases the spread of the infection.

I think the problem Fauci has is that he is confident that it is better to provide an answer than to hedge. It would be pretty reasonable for him to say something about getting vaccinations done as quickly as possible and evaluating what other measures are necessary once a larger percentage of the population is vaccinated.