We shouldn't ignore those who have been infected and there's a fairly wide range of estimates on that
People are not acting normally across the US. Mobility is down and people are taking precautions. There is no hard line between when herd immunity is achieved and herd immunity isn't achieved.
Young kids potentially being less susceptible to infection and less productive spreaders and non homogeneous population mixing may lower the herd immunity threshold a bit from a simple Ro calculation.